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to NBS SPECIAL PUBLICATION 589

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Eddy Current

Nondestructive Testing
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act ot Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

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the Institute for Computer Sciences and Technology.

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provides engineering measurement traceability services; develops test methods and proposes

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2 Some divisions within the center are located at Boulder, CO 80303.
National Bureau of Standards

Library, £-01 Admin. Bldg,

FEB 2 7 1981

not o.cc. -£j <c Eddy Current Nondestructive Testing

no. S~$9

C. 3~

Proceedings of the Workshop on

Eddy Current Nondestructive Testing,

held at the National Bureau of Standards,

Gaithersburg, Maryland, on November 3-4, 1977

Edited by:

George M. Free

Center for Absolute Physical Quantities

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

\

CO J

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U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

j NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued January 1981
Library of Congress Catalog Card Number: 80-600172

National Bureau of Standards Special Publication 589

Nat. Bur. Stand. (U.S.), Spec. Publ. 589, 153 pages (Jan. 1981) CODEN. XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981

For sale l>y the Superintendent of Documents, U.S. Government Printing Office Washington. D.C. 20402 - Price $5.50
FOREWORD

Although testing with eddy currents is regarded as one of the major

methods for nondestructive inspection,

many people in the industry regard

this technique as one that offers much

greater potential than is presently

realized. It is now used primarily

for the sorting of alloys by conductivity measurements and for the inspection of relatively thin conducting material; thin-walled tubing constitutes a major inspection item for eddy current

techniques.

In the Nondestructive Evaluation

(NDE) Program at the National Bureau of

Standards, we are working to improve the

reliability of nondestructive measurements. The present effort in eddy current testing is directed primarily at

conductivity measurements; a measurement

service and standard reference materials

are planned to help the industry improve

this type of NDE measurements. Looking

beyond that, however, we at NBS agree

that new ideas and developments can lead

to greater utilization of eddy current

methods. One means to examine that

potential was a Workshop on Eddy Current

Nondestructive Testing; the Workshop was

held at NBS on November 3 and 4, 1977,

under the joint sponsorship of the NBS

Electricity Division and the NDE Program.

These Proceedings are a record of that

Workshop.

The purposes of the Workshop were to

(1) review the current status of eddy cur- rent measurement methodology and applications, (2) define the directions for improved techniques and applications, and

(3) assess the needs for work on standards

and underlying science to address present

and future problems. The attendees were drawn from industry, university, and government. We have thanked them all individually, but it is appropriate here also

to express our appreciation to them again

and particularly to the speakers.

I also wish to express my appreciation to the planners of the Workshop,

Norman Belecki, George Free, and Barry

Taylor of the NBS Electricity Division?

George Birnbaum, of the NDE Program; and

Robert Green of the Johns Hopkins University. I am confident that these Proceedings will serve their intended purposes and help the industry and NBS define fruitful areas for additional work

to improve eddy current nondestructive

testing.

Harold Berger

Program Manager

Nondestructive Evaluation

February 1978

iii

PREFACE

The intent of these Proceedings is to provide a record of the NBS Workshop

on Eddy Current Nondestructive Testing.

With the excpetion of the first paper,

an overview of eddy current testing by

Dr. Robert McMaster, each paper presented was followed by a period of discussion. The Proceedings followed the

same format. Unfortunately, the com- ments of participants could not be at- tributed in all cases, but where it is

possible the authors of the many com- ments, questions, and ideas are noted.

Some editing of the discussion periods

was done, consequently the discussion

periods are not "verbatim."

Due to the method of printing the proceedings, not all pictures and diagrams

turned out to be of equal clarity. I apologize beforehand to those authors whose

pictures or diagrams are not of the excellent quality which the participants viewed

at the workshop.

George M. Free

Editor

v

TABLE OF CONTENTS

PAGE

FOREWORD iii

Harold Berger

PREFACE v ABSTRACT vi i i

THE HISTORY, PRESENT STATUS, AND FUTURE DEVELOPMENTS OF EDDY CURRENT TESTS 1 Robert C. McMaster

EDDY CURRENT TESTING: PRESENT AND FUTURE APPLICATIONS IN THE FERROUS METAL INDUSTRY 33

Richard B. Moyer

EDDY CURRENT STANDARDS IN NONFERROUS METALS 39

Carlton E. Burley

EDDY CURRENT INSPECTION OF GAS TURBINE BLADES 45

Robert A. Betz

EDDY CURRENT EXAMINATION IN THE NUCLEAR INDUSTRY 49

Allen E. Wehrmeister

EDDY CURRENT INSPECTION SYSTEMS FOR STREAM GENERATOR TUBING IN NUCLEAR POWER PLANTS 57 Clyde J. Denton

USE OF ROUND ROBIN TESTS TO DETERMINE EDDY CURRENT SYSTEM PERFORMANCE 63

E. R. Reinhart

EDDY CURRENT TESTING IN GOVERNMENT 77 Patrick C. McEleney

CURRENT STATUS AND FUTURE DIRECTIONS OF EDDY CURRENT INSTRUMENTATION 81

Tracy W. McFarlan

MULTIFREQUENCY EDDY CURRENT INSPECTION TECHNIQUES 87

Hugo L. Libby

PULSED EDDY CURRENT TECHNIQUES FOR NONDESTRUCTIVE EVALUATION . 107

D. L. Waidelich

THE INTRODUCTION OF SIGNAL PROCESSING TECHNIQUES TO EDDY CURRENT INSPECTION 113

E. E. Weismantel

DEVELOPMENT OF NON-FERROUS CONDUCTIVITY STANDARDS AT BOEING 121

Arthur Jones

NBS EDDY CURRENT STANDARDS PROGRAM 133

George Free

ELECTROMAGNETIC THEORY AND ITS RELATIONSHIP TO STANDARDS 137

Arnold H. Kahn and Richard D. Spal

DISCUSSION: NEW SCIENCE DIRECTIONS AND OPPORTUNITIES CONCLUDED FROM WORKSHOP SESSIONS 143

Leader: Robert E. Green

DISCUSSION: NEW DIRECTIONS FOR STANDARDS CONCLUDED FROM WORKSHOP SESSIONS 149

Leader: Norman B. Belecki

SPECIAL ACKNOWLEDGEMENTS 155

LIST OF WORKSHOP PARTICIPANTS 157

vi i
ABSTRACT

The proceedings of the Eddy Current Nondestructive Testing Workshop held at NBS in

November 1977 contain papers related to all areas of eddy current testing. A historical

overview of the discipline from its inception until the present is given. Other papers

discuss the use of eddy current testing in the primary metals industry (both ferrous and

nonferrous metals), the use of eddy currents for the sorting of metals and for defect detection, the state-of-the-art in eddy current instrumentation, and the use of signal processing in the analysis of eddy current signals. The development and use of eddy current

standards is discussed as well as several of the newer areas of eddy current development,

i.e., multi frequency and pulsed eddy current techniques.

Key words: Conductivity; defect detection; eddy current test; mul tifrequency; nondestructive testing.

viii
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977. Issued

January 1981.

THE HISTORY, PRESENT STATUS, AND FUTURE DEVELOPMENT OF EDDY CURRENT TESTS

Robert C. McMaster

Departments of Electrical Engineering and Welding Engineering

The Ohio State University

Columbus, OH

1. Historical Development of Eddy Current

Theory and Test Methods

It is probable that no other form of

nondestructive testing has a history of

illustrious scientific creativity and

practical development that compares with

the past century and a half of development

of the concepts and applications of electromagnetic induction and eddy current

testing. James Clerk Maxwell, in his

remarkable two- volume work A Treatise on Electricity and Magnetism , published in

several editions from 1873 to 1891, sum- marized the first half century of this

history [l] 1. In addition, he conceived

and published the comprehensive group of

relations known as Maxwell's equations for

the electromagnetic field, which mathematically represent almost the entire

present knowledge of this subject. For the

past hundred years, physicists and re- searchers in electricity and magnetism have

occupied themselves with numerous applications of Maxwell's theory. However,

during this past century, no one has

conceived any significant new law to be

added to Maxwell's principles (with the

possible exception of Einstein's theory of

relativity, which extends the theory of the

electromagnetic field to a fourdimensional framework of three spatial

dimensions and a fourth dimension of

time). NOTE: In the following segments

abstracted from Maxwell's treatise, the

symbol .... indicates omissions. Parentheses are used to indicate explanatory

words or comments inserted by the author of

this paper. Superscript numbers following

headings identify the specific articles of

Maxwell's treatise used as sources.

2. Oersted's 1820 Discovery of the Magneti

Field of an Electric Current

(475_478)

As described by Maxwell, "....Conjectures of various kinds had been made as to the relation between electricity and

magnetism, but the laws of these phenomena,

and the form of these relations, remained

entirely unknown till Hans Christian

Oersted, at a private lecture to a few

advanced students at Copenhagen, observed

that a wire connecting the ends of a voltaic battery affected a magnet in its

vicinity. This discovery he published in a tract. . .dated July 21, 1820 [2].

....Oersted discovered that the current

itself was the cause of the action, and

that the 'electric conflict acts in a revolving manner', that is, that a magnet

placed near a wire transmitting an electric

current tends to set itself perpendicular

to the wire, and with- the same end always

pointing forwards as the magnet is moved

around the wire.... The space in which

these forces act may therefore be con- sidered as a magnetic field.... In the case of an indefinitely long straight wire

carrying an electric current. ... the lines

of magnetic force are everywhere at right

angles to planes drawn through the wire,

and are therefore circles each in a plane

perpendicular to the wire, which passes

through the wire." (Had Oersted been

provided with a much larger current, it is

possible that even a piece of nonmagnetic

conducting metal lying adjacent to the

current-carrying loop would have reacted to sudden application of the current, by eddy

current reaction. Had this accident oc- curred, it is possible that the discovery

of the effects of eddy currents might

possibly have occurred more than 150 years

ago.

)

figures in brackets indicate literature references at the end of this paper.

1
3. Ampere's 1820 Discovery of the Mutual

Interaction of Two Currents^^

Maxwell continues "...The action of one circuit upon another was originally

investigated in a direct manner by Ampere

almost immediately (in 1820) after the

publication of Oersted's discovery. .. .Am- pere's fundamental experiments are all of

them examples of.... the null method of

comparing forces.... In the null method,

two forces, due to the same source, are made to act simultaneously on a body

already in equi 1 ibrium. . . . No effect is

produced, which shows that these forces

are themselves in equilibrium. This

method is peculiarly valuable for com- paring the effects of the electric current when it passes through circuits of

different forms. By connecting all the

conductors in one continuous series, we ensure that the strength of the current

is the same at every point of its course.... Since the current begins

everywhere throughout its course almost

at the same instant, we may prove that

the forces due to its action on a sus- pended body are in equilibrium by observing that the body is not at all affected by the starting or the stopping

of the current.

"Ampere's balance consists of a light frame capable of revolving about a vertical axis, and carrying a wire which

forms two (rectangular loop) circuits of

equal area, in the same plane or in parallel planes, in which (loops) current

flows in opposite directions. The object

of this arrangement is to get rid of the

effects of terrestrial magnetism on the

conducting wire.... By rigidly connecting

two circuits of equal area in parallel

planes, in which equal currents run in opposite directions, a combination is formed which is unaffected by terrestrial

magnetism. ... (This balance) is therefore

called an Astatic Combination (see fig.

1). It is acted upon, however, by forces

arising from currents or magnets which

are so near to it that they act differently on the two circuits Am- pere's theory of the mutual action of

electric currents is founded on four ex- perimental facts and one assumption."

Ampere's 1820 experiments provided

several useful techniques employed in

present-day eddy current test systems,

including:

1. The methods of shielding leadwire connections to test coils.

2. The use of comparison-coil

arrangements to reduce or eliminate test

signal components related to common properties of two test objects.

3. The use of differential-coil

arrangements to compare local differences

in properties of adjacent areas of a single test object.

4. The methods of coupling

magnetizing-coil fields with test ma- terial surfaces.

5. The use of dual -coil systems to balance out external magnetic and electric field effects (including terrestrial

magnetism).

Figure 1. Maxwell's sketch illustrating

Faraday's basic test arrangement with

astatic balance coil arrangement [1].

(It would be difficult to estimate how

many hundreds of 20th century patents are based upon these simple discoveries by

Ampere a hundred years earlier. It would

also be difficult to estimate the man- years of effort and dollar costs lost

during recent developments of eddy cur- rent test systems by workers who were not aware of the full significance of

Ampere' s 1820 work.

)

3.1 Ampere's first experiment

(Shielding of lead wires)^5 ^

In Maxwell's words, "Ampere's first

experiment is on the effect of two equal

currents close together (flowing) in

opposite directions. A wire covered

with insulation is doubled upon itself

and placed near one of the circuits of

the astatic balance (See fig. 1). When a current is made to pass through the

(looped-back) wire and (the compensated

loops of) the balance, the equilibrium of

the balance remains undisturbed, showing

that two equal currents close together in

opposite directions neutralize each other.

2
If, instead of two wires side by side, a wire be insulated in the middle of a metal

tube, and if the current pass through the

wire and back by the tube, the action

out'-ide the tube is not only approximately

but accurately null. This principle is of

great importance in the construction of

electric apparatus, as it affords the means of conveying the current to and from

any galvanometer or other instrument in

such a way that no electromagnetic effect

is produced by the current on its passage

to and from the instrument. In practice,

it is generally sufficient to bind the

wires together, care being taken that they

are kept perfectly insulated from each

other, but where they must pass near any

sensitive part of the apparatus it is

hotter to make one of the conductors a tube and the other a wire inside it."

( i'hese techniques, including also twisted

lead pairs, are commonly used to connect

instruments to sensing coils or semiconductor detectors used today to detect eddy

current magnetic field test signals. At

higher frequencies, shielding by concentric conductors (usually grounded at one

end) aids in avoidance of interfering

signals from ambient electromagnetic

fields or moving ferromagnetic machine

parts or test objects.)

3.2 Ampere's second experiment (Effect of

A + + k N (506) crooked current paths) v '

Maxwell reports: "In Ampere's second

experiment one of the wires is bent and

crooked with a number of small sinuosities, but so that in every part of its course it remains very near the straight

wire. (See fig. 1) A current flowing

through the crooked wire and back again

through the straight wire, is found to be

without influence upon the astatic balance.

This proves that the effect of the current

running through any crooked part of the

wire is equivalent to the same current

running in the straight line joining its

extremities, provided the crooked line is

in no part of its course far from the

straight one. Hence any small element of

a circuit is equivalent to two or more component elements, the relation between

the component elements and the resultant

element being the same as that between

component and resultant displacements or velocities." (This basic principle has

been generally ignored with respect to its

significance in detection of small discontinuities that locally distort eddy

current flow paths. A circular test

coil, for example, produces a mirror-image

circular flow path of eddy currents in the

adjacent test material. Small diversions

and excursions of eddy currents from a truly circular path will have very small

effects upon signal pickup coils coincident with the magnetizing coils. Local

detectors of distortions of the eddy

current magnetic field can have far

greater sensitivity to small discontinuities than large-area pickup coils.)

3.3 Ampere's third and fourth

QVnQV,. on . (507-509, 520-521) experiments v

' '

Ampere's third experiment demonstrated that external currents or magnets

had no tendency to move a straight

current-carrying conductor in the direction of its length (see fig. 2). The

fourth experiment showed that the force

Figure 2. Maxwell's sketch illustrating Faraday's third experiment showing

no force acting along the length of a current carrying conductor.

acting between two adjacent currentcarrying loops varies as the square of the

distance between the two loops. Analyses

of these results indicates that the mutual

potential M of two closed circuits carrying unit current expresses the work done

by electromagnetic forces on either

conducting circuit when it moves parallel

to itself from an infinite distance to its

actual position. Any alteration of its

position, by which M is increased, will be

assisted by the magnetic forces. Even

when the motion of the circuit is not

parallel to itself, the forces acting on

it are still determined by the variation

of M, the potential of one circuit on the

other. The force between the circuits is dM/dx, and thus M, is related to the

energy of the electromagnetic field of the

current-carrying circuits.
4. Faraday's 1831 Discovery of the Law of

ci + +• t a *• (528-541) Electromagnetic Inductiorr

Maxwell notes that: "The discovery

by Oersted of the magnetic action of an electric current led by a direct process

of reasoning to that of magnetization by

electric currents, and of the mechanical

action between currents. It was not,

however, till 1831 that Faraday, who had

been for some time endeavouring to produce electric currents by magnetic or electric action, discovered the conditions of magneto-electric induction. The

method which Faraday employed in his

researches consisted of a constant appeal

to experiment as a means of testing the

truth of his ideas, and a constant cultivation of ideas under the direct influ- ence of experiment. Faraday. ... shows us his unsuccessful as well as his successful experiments, and his crude ideas as well as his developed ones. The reader,

however inferior to him in inductive

power, feels sympathy even more than

admiration, and is tempted to believe

that, if he had the opportunity, he too would be a discoverer. Every student

should therefore read Ampere's research

as a splendid example of scientific style

in the statement of a discovery, but he

should also study Faraday for the cultivation of a scientific spirit, by means

of the action and reaction which will

take place between the newly-discovered

facts as introduced to him by Faraday and

the nascent ideas of his own mind.

"The method of Faraday seems to be

intimately related to the method of

partial differential equations and integrations throughout all space.... He never considers bodies as existing with

nothing between them but their distance,

and acting upon one another according to some function of that distance. He conceives all space as a field of force,

the lines of force being in general

curved, and those due to any body ex- tending from it on all sides, their

directions being modified by the presence

of other bodies. He even speaks of the

lines of force belonging to a body as in some sense part of itself, so that in its

action on distant bodies it cannot be

said to act where it is not. This,

however, is not a dominant idea with

Faraday. I think he would rather have

said that the field of space is full of

lines of force, whose arrangement depends

on that of the bodies in the field, and

that the mechanical and electrical action

on each body is determined by the lines

which abut on it.

4.1 Faraday's law for induction by variation

of primary current^^'

Maxwell advises the reader to read

Faraday's "Experimental Researches,

Series i and ii," and then summarizes

four forms of Faraday's law of induction

His description of the first form of

Faraday's law follows:

"Let there be two conducting circuits, the Primary and the Secondary

circuit. The primary circuit is con- nected with a voltaic battery by which

the primary current may be produced,

maintained, stopped, or reversed. The

secondary circuit includes a galvanometer

to indicate any currents which may be

formed in it. This galvanometer is

placed at such a distance from all parts

of the primary circuit that the primary

current has no sensible direct influence

upon its indications.

"Let part of the primary circuit

consist of a straight wire, and part of

the secondary circuit of a straight wire

near and parallel to the first, the other

parts of the circuits being at a greater

distance from each other.

"It is found that at the instant of

sending a current through the straight

wire of the primary circuit the galvanometer of the secondary circuit indicates

a current in the secondary straight wire

in the opposite direction. This is

called the induced current. If the

primary current is maintained constant,

the induced current soon disappears, and

the primary current appears to produce no effect on the secondary circuit. If now

the primary current is stopped, a secon- dary current is observed, which is in the same direction as the primary current.

Every variation of the primary current

produces electromotive force in the

secondary circuit. When the primary

current increases, the electromotive

force is in the opposite direction to the current. When it diminishes, the electromotive force is in the same direction

as the current.

"These effects of induction are increased by bringing the two wires nearer

together. They are also increased by

forming them into two circular or spiral

coils placed close together, and still more by placing an iron rod or a bundle

of iron wires inside the coils."
(This experiment demonstrates the

fundamental principles of the use of

magnetizing coils in eddy current testing. The need for a time-varying primary

current is clearly indicated. The advantage of close coupling or spacing

between the magnetizing coil and test

metal surface is also shown. This

translates into control of lift-off of

probe coils, and preference for high

coil-fill factors with encircling-coil

eddy current tests. The need for pulsating or alternating primary current is

also now evident. Finally, the advantages of using ferrite or iron cores in

eddy current probe coils are suggested.

Present-day eddy current test systems

make full use of each of these principles, enunciated clearly by Faraday in

1831.)

4.2 Faraday's law for induction by motion

of the primary circuit^"^

"We have seen that when the primary

current is maintained constant and at rest the secondary current rapidly disappears. Now, let the primary current be

maintained constant, but let the primary

straight wire be made to approach the

secondary straight wire. During the

approach, there will be a secondary

current in the opposite direction to the

primary. If the primary circuit be moved

away from the secondary, there will be a secondary current in the same direction

as the primary.

(Two principles are implied by the

concept of induction by motion of the

primary circuit. The first is that polarized and directional secondary currents can be induced by moving a straightline primary current over a conducting

test surface. Secondly, alternating

current could be induced in a conducting

secondary circuit or test material when a constant-current primary coil is moved

cyclically up and down or side to side

over a secondary coil or conducting test

surface. Where scanning eddy current

tests are required, it is possible that a permanent magnet or a direct-current

magnetizing coil could be used to induce

eddy currents, without the need for an electronic oscillator or ac power supply.

An additional concept implied by this

technique of induction would be that of

using dc magnetic field detectors to measure the magnitude of secondary current or eddy currents in a conducting

material, under or lagging behind the

moving primary coil. The decay rate of

dc current measured at a fixed distance

behind the moving primary coil. The

decay rate of dc current measured at a fixed distance behind the moving primary

coil would contain information similar to phase and amplitude data obtained by

phase-plane analysis of ac eddy current

test systems in common use today. Of

course, this type of system would perhaps

best be used with very rapid scanning

over test surfaces.)

4.3 Faraday's law for induction by

motion of the secondary circuit^*^

Maxwell states also: "If the secon- dary circuit be moved, the secondary

current is opposite to the primary when

the secondary wire is approaching the

primary wire, and in the same direction

when it is receding from it. In all cases, the direction of the secondary

current is such that the mechanical

action between the two conductors is

opposite to the direction of motion,

being a repulsion when the wires are approaching, and an attraction when they

are receding. This very important fact was established by Lenz."

(This example suggests that a rapidly-moving conducting test material

such as sheet metal in a rolling mill

could pass by a stationary test coil

carrying direct current which induces

flow of current in material both approaching and leaving the area of this

local magnetization. Detectors of the

eddy current field in either location

could respond to local discontinuities or variations in material properties which

influence the amplitude and distribution

of the eddy currents.)

4.4 Faraday's law for induction by the

relative motion of a magnet and the

secondary circuit^*^

Maxwell continues with: "If we substitute for the primary circuit a magnetic shell, whose edge coincides with

the circuit, whose strength is numerically equal to that of the current in the

circuit, and whose austral face corresponds to the positive face of the circuit, then the phenomena produced by the

relative motion of this shell and the

secondary circuit are the same as those

observed in the case of the primary circuit. " (The coil of the preceding
examples could be replaced by a permanent

magnet when relative motion exists between

the magnet and test material in eddy cur- rent tests, providing adequate secondary

current magnitude and speed of motion can

be attained.

)

4.5 Summary expressions for Faraday's

n * • A 4.- (531,534,536) law of -induction v

' ' '

Maxwell summarizes the various

statements of Faraday's law with the

following statements: "When the number of

lines of magnetic induction which pass

through the secondary circuit in the

positive direction is altered, an electromotive force acts round the circuit,

which is measured by the rate of decrease

of the magnetic induction through the

circuit. .. .The intensity of the electromotive force of magneto-electric induction

is entirely independent of the nature of

the substance of the conductor in which it

acts, and also of the nature of the con- ductor which carries the inducing current.... The electromotive force of the

induction of one circuit on another is

independent of the area of the section of

the conductors. .. .The electromotive force

produced in a coil of n windings by a current in a coil of m windings is

proportional to the product mn. . .

.

Maxwell finally states the "true law

of magneto- induction" in the following

terms: "The total electromotive force

acting around a circuit at any instant is

measured by the rate of decrease of the

number of lines of magnetic force which

pass through it. When integrated with

respect to time, this statement becomes:

The time integral of the total electromotive force acting round any circuit,

together with the number of lines of

magnetic force which pass through the

circuit, is a constant quantity. ... This

quantity may even be called the fundamental quantity in the theory of electromagnetism. Faraday. ... recognized in the

secondary circuit, when in the electromagnetic field, a 'peculiar electrical

condition of matter' to which he gave the

name of the Electrotonic State."

(This quantity being defined as of

most fundamental nature appears to be

similar to the concept of 'flux linkages',

measured by the product of the number of

winding turns and the total magnetic flux

enclosed in the winding, N0. This

quantity is also expressed by the term MI,

where M is the potential of the coupled

circuits, and I is the current in any coil

winding.

)

5. Lenz's 1834 Law Showing Effects

Opposing Causes in Electromagnetic

Induction (542)

Maxwell's narrative of the development of basic electromagnetic theory

continues its description of the early

years of development as follows: "In

1834, Lenz enunciated the following

remarkable relation between the phenomena

of mechanical action of electric currents, as defined by Ampere's formula,

and the induction of electric currents by i the relative motion of conductors....

Lenz's law is as follows:

"If a constant current flows in the

primary circuit A, and if, by the motion

of A, or of the secondary circuit B, a current is induced in B, the direction of

this induced current will be such that,

by its electromagnetic action on A, it

tends to oppose the relative motion of

the circuits.

(Stated more generally, Lenz's law

states that the electromagnetic field

will act so as to oppose or resist any

effort made to change its intensity or configuration. Where mechanical motion

causes the change, mechanical force | developed within the system will oppose

1 the change. If mechanical motion is

absent, electromotive forces will be

induced which tend to maintain the status

quo, namely to maintain the total fluxj

linkages in the system.)

6. Neumann's 1845 Develpment of

(S Mathematical Theory of Induction v

Maxwell's history of developments

continues with: "On (Lenz's) law, F. E. Neumann founded his mathematical theory,

of induction in which he established the

mathematical laws of the induced currents;

due to motion of the primary or secondary | conductor. He showed that the quantity M

.... is the same as the electromagnetic

potential of one circuit on the other....

We may regard F. E. Neumann, therefore,

as having completed for the induction of

currents the mathematical treatment which

Ampere had applied to their mechanical

action.
7. Helmholtz 1847 Derivation of Laws of

(543 Induction From Conservation of Energy^

In Maxwell's opinion: "A step of

still greater scientific importance was

soon after made by Helmholtz in his

'Essay on the Conservation of Force, 1 and

by Sir William Thompson, working somewhat

later, but independently of Helmholtz.

They showed that the induction of electric currents discovered by Faraday could

be mathematically deduced from the electromagnetic actions discovered by Oersted

and Ampere by the application of the

principle of Conservation of Energy.

8. Faraday's Recognition of Electromagnetic

Kinetic Energy and Momentun/^

Maxwell reports that: "Faraday

showed that (the phenomenon of selfinduction) and other phenomena which he

describes are due to the same inductive

action which he had already observed the

current to exert on neighboring conductors. In this case, however, the inductive action is exerted on the same con- ductor which carries the current, and it

is so much the more powerful as the wire

itself is nearer to the different elements of the current than any other wire

can be. He observes, however, that 'the

first thought that arises in the mind is

that the electricity circulates with

something like momentum or inertia in the

wire. ' Indeed, when we consider one particular wire only, the phenomena are exactly analogous to those of a pipe full

of water flowing in a continued stream.

If while the stream is flowing we suddenly close the end of the pipe, the

momentum of the water produces a sudden

pressure, which is much greater than that

due to the head of water and may be

sufficient to burst the pipe. . .

.

"These results show clearly that, if

the phenomena are due to momentum, the

momentum is certainly not that of the

electricity in the wire, because the same

wire, conveying the same current, ex- hibits effects which differ according to

its form; and even when its form remains

the same, the presence of other bodies

such as a piece of iron or a closed

metallic circuit, affects the result."

(This latter effect is that involved in

eddy current testing.)

"It appears, therefore, that a system containing an electric current is

a seat of energy of some kind; and since

we can form no conception of an electric

current except as a kinetic phenomenon,

its energy must be kinetic energy, that

is to say, the energy which a moving body

has by virtue of its motion.

"We have already shown that the

electricity in the wire cannot be con- sidered as the moving body in which we

are to find this energy, for the energy

of a moving body does not depend upon

anything external to itself, whereas the

presence of other bodies near the current

alters its energy.

9. Influence of Faraday's Research Upon

19th Century Inventors

Michael Faraday's two-volume work

"Experimental Researches in Electricity"

influenced numerous investigators and

inventors in Europe and the United States

from the 1830' s to the end of the nineteenth century. This led many others to experiment with electromagnetic effects

and to develop many basic inventions such

as Morse's telegraph, Bell's telephone,

and Edison's many improvements on telegraphic, telephonic, fire alarm, and

stock ticker communication systems.

Faraday in 1831 also showed before the

Royal Society a homopolar generator (a

disc rotating between the poles of a large horseshoe magnet) for converting

mechanical energy into electric energy.

His influence upon inventors with little

or no scientific training was very great,

for Faraday's accounts of his experiments

did not use any complicated mathematical

formulas. A biographer of Thomas Edison

notes that Faraday appeared to be the

Master Experimenter whose laboratory

notes communicated the highest intellectual excitement--and hope as well.

Faraday's explanations were simple,

steeped in the spirit of truthfulness and

humility before nature. For Faraday, the

natural laws were revealed through ex- periment. To American inventors, Faraday, poor and self-educated, indifferent

to money or titles, exemplified the

ethics of a true man of science, whom

they could emulate. Thus, during the

period from 1831 to about 1875, the

inventions made on the basis of Faraday's

research were often developed by trial

and error, empirically, and step-by-step.

7
10. Maxwell's Proposal for Development of

(552^ Theory of the Electromagnetic Field v '

Based upon the facts previously

summarized in this introduction, James

Clerk Maxwell outlines his plan for

developing a unified theory of the electromagnetic field, as follows:

"We are therefore led to inquire

whether there may not be some motion

going on in the space outside the wire,

which is not occupied by the electric

current, but in which the electromagnetic

effects of current are manifested.

"I shall not at present enter on the reasons for looking in one place rather

than another for such motions, or for

regarding these motions as of one kind

than another.

"What I propose now to do is to examine the consequences of the assumption that the phenomena of the electric

current are those of a moving system, the

motion being communicated from one part

of the system to another by forces, that

nature and laws of which we do not even attempt to define, because we can eliminate these forces from the equations of

motion by the method given by Lagrange

for any connected system.

" 1 propose to deduce the main

structure of the theory of electricity

from a dynamical hypothesis of this kind,

instead of following the path which has

led Weber and other investigators to many

remarkable discoveries and experiments,

and to conceptions, some of which are as beautiful as they are bold. I have

chosen this method because I wish to show

that there are other ways of viewing the

phenomena which appear to me more satisfactory, and at the same time are more consistent with the methods followed in

the preceding parts of this book than

those which proceed on the hypothesis of

direct action at a distance."

11. Maxwell's Equations for Electric

Circuits and for Electromagnetic

Fields (578-619)

Maxwell's remarkable achievement of

integrating the available knowledge con- cerning electromagnetic circuits and

fields provides the basis for analysis of

all basic eddy current and electromagnetic induction problems—and for most of

modern electromagnetic theory. These

simple equations in both integral and

differential form were derived by the

methods of Lagrange, using relationships

from the calculus of variations. Solutions for alternating fields are also

available for many configurations of the

fields.

It is of interest that simpler

techniques, using an 'operational map'

have been devised by the author for

presenting these types of equations and

their derivations in simple form for use by second-year engineering students.

Since the equations are available in

nearly all basic textbooks on the electromagnetic field, they will not be

repeated here. Lord Kelvin devised the

solutions of Bessel's equation for the cases of probe coils, for example, and

provided the so-called Kelvin functions

from which simple cases can be readily

calculated by hand or by digital computers.

11.1 Development of practical electromagnetic induction test methods

It has been reported that Hughes

demonstrated the basic features of eddy

current nondestructive testing in the

1860's showing that it was possible to differentiate between metallic conducting

coins by a simple arrangement of magnetizing coil and induction of eddy currents

in the coins.

12. Early Tests for Eddy Current and

Hysteresis Losses in Electrical Steel

Sheets

Active practical interest in use of

electromagnetic means for sorting of

metals and detection of discontinuities

did not result in many useful test devices

prior to the beginning of the twentieth

century. However, the numerous developments including that of alternating cur- rent electric power systems, and the use of transformers and other induction

machines, provided a base of practical

design and a need to investigate the

losses occurring in magnetic core materials used in these devices. Much

effort was devoted to reduction of eddy

current and magnetic hysteresis losses in

laminated steel sheets, particularly by

addition of silicon and other alloying

elements which lowered their electrical

conductivity and use of purer iron alloys

with, in some cases, directional rolling

to attain maximum permeability and minimum

hysteresis losses.
To a first approximation, in cores

formed of thin magnetic laminations, it was shown that eddy current losses tended

to increase in proportion with the square

of the frequency, and hysteresis losses

in accordance with the 1.6th power of the

frequency of alternation of the magnetic

field intensity. Numerous laboratories,

including those of electrical equipment

manufacturers such as Westinghouse and

The General Electric Company, and of

manufacturers of electrical steel sheets

such as Al legheny-Ludlum and Armco Steel

Company, established measurement laboratories to monitor properties of production steel sheets and assure specified

electromagnetic loss factors for electrical steel sheets. The well-known

Epstein test, and many others, were used

for these material tests.

Many improvements resulted, including use of thinner sheets, use of oriented steel sheets, and use of insulating

coatings between sheets to limit eddy

current flow paths. Also discovered

during these magnetic core improvements

were the undesirable effects of mechanical clamping stresses and stresses

resulting from punching and shearing of

laminations, which tended to increase

core losses under ac excitation. Hydrogen annealing and other techniques, such

as those developed by Dr. Trigvie Yensen

of Westinghouse Research Laboratories,

led to improved materials such as Hypersil, Hypernik, and other magnetic sheet

alloys with superior properties. Control

of other alloying elements, additions of

up to 50% nickel, and orientation of

grain structures and magnetic domains were used to develop special steels with

rectangular hysteresis loops which are used in magnetic switching of electrical

currents, saturable reactors and magnetic

amplifiers, and many novel electromagnetic devices. These developments illustrated the variations in electrical

conductivity, magnetic permeability,

grain orientation and anisotropy, me- chanical stresses, alloy contents, and

impurity contents, which influenced the

electromagnetic response of ferromagnetic

materials and changed the apparent inductance and resistive losses measured by

their magnetizing coils. The use of

direct-current bias to adjust the apparent inductance in saturable reactors

and transductors for power control purposes also illustrated a means for re- ducing magnetic permeability and incremental inductance or inductive reactance.

It was also observed that many magnetic

core materials introduced odd harmonics

into the magnetizing currents or voltages

across inductances of their magnetizing

coils (or into unloaded secondary windings on the cores), and the high sensitivity of the harmonic signals to material conditions and mechanical stressing

were known and purposely avoided where

possible.

These various effects, well-known to

electrical designers at the turn of the

century, have since become possible

methods for control or read-out of eddy

current nondestructive test signals.

(However, in general, the highlypermeable electrical steel sheets now commercially-available are not ideal for

eddy current tests since their eddy

current losses are so very low. For

their evaluation, electromagnetic induction tests responsive primarily to hysteresis effects, including higher harmonic effects, may prove more useful.)

13. Development of Techniques for Analysis

of Inductive ac Electrical Circuits

The sinusoidal oscillations of

alternating-current electric power

system voltages and currents introduced

new complexities in analysis of circuit

performance, as compared with analyses

for Edison's earlier direct-current

electric power systems. As early as 1893, Professors Crehore and Beddell of

Cornell Univeristy prepared a textbook of

analysis of ac electric circuits, including effects of resistive, capacitive,

and inductive circuit elements. This

book was based upon detailed solution of

the differential equations developed by

Maxwell, and involved use of calculus in

each solution. Soon thereafter, Steinmetz came to the United States with the

Thomson-Houston Company (later General

Electric Company) and he developed much

simplified methods of analysis using

rotating line segments which he called

"vectors" (now called sinors) to represent sinusoidal quantities. As such line

segments rotated about one end (at the

origin of coordinates), their vertical

projections mapped out the ordi nates of

the sinusoidal waves, when these vertical

projections were plotted as functions of

time. Together with the technique of

representing impedances on a complex

plane (with resistance R as a horizontal

coordinate, and inductive reactance, X , as a vertical coordinate), the use of

these phasor quantities reduced the

9
solutions for steady state alternating

currents to simple algebra and trigonometry, rather than integral calculus.

These methods of signal analysis on the

complex plane are widely used today in

analysis of eddy current tests, following

their clear enunciation by Dr. Friedrich

Forster of West Germany, following World

War II.

14. Early Industrial Development of

Electromagnetic Induction Comparators

Numerous electromagnetic induction

or eddy current comparators were patented

in the United States in the period from

1925 until the end of World War II in

1945. Many of these were referenced in

1950 by McMaster and Wenk in an ASTM

publication, updating a prior (1948)

summary of basic nondestructive test

methods. (See Tables I, II and Appendix

I.) Innumerable examples of comparator

tests were reported in the literature and

in patents. Many provided simple com- parator coils into which round bars or other test objects were placed, producing

simple changes in amplitudes of test

signals, or unbalancing simple bridge

circuits. In nearly all cases, and

particularly where ferromagnetic test

materials were involved, no quantitative

analyses of test-object dimensions,

properties, or discontinuities were possible with such instruments. Often,

difficulties were encountered in reproducing test results, since some test

circuits were adjusted or "balanced" to optimize signal differences between a "known good test object" and a "known

defective test object," for each group of

objects to be tested. Little or no correlation could then be obtained between various types of specimens, each

type having been compared to an arbitrarily-selected specimen of the same specific type.

Many simple comparators operated on 60 Hz alternating current from 110 volt

ac circuits, using conventional instruments such as voltmeters, ammeters,

wattmeters and, occasionally phase meters. Such meters typically absorbed

energy from the test circuits, and had

typical accuracies and reproducibilities

often of only 1% or 2% of full-scale

readings. In other cases, well-known

Wheatstone bridge circuits were employed

to balance out comparison test arrangements, and to provide greater sensitivity

to signal differences. For the most

part, many of these early comparator

systems were short-lived, and received

little acceptance in industry. By com- parison, a few such developments, sponsored by major industries or persistent

creative inventors who sought support and

set up their own companies, survived and

are used in their modernized form in

American industry today.

14.1 1925-1945 American developments of

electromagnetic tests for steel products

Examples of continuing development

of electromagnetic induction tests for

use in inspection of round bars, tubes,

billets, and products of the steel industry of the United States were those of

Magnetic Analysis Corporation and Republic Steel Corporation. Both are based

upon the continuing efforts of a few

dedicated individuals who passed their

skills and enthusiasms along to their successors in the same development

organizations. Charles W. Burroughs,

Carl Kinsley, and Theodore W. Zuschlag

were among the pioneers of the Magnetic

Analysis Corporation, whose test products

are still commercially available in 1978.

Archibald H. Davis, Horace G. Knerr, and

Alfred R. Sharpies received basic patents

for Steel and Tubes, Inc. (now Republic

Steel Corporation). Their developments

were extended and continued in the Electromechanical Research Laboratory of

Republic Steel in Cleveland by Cecil

Farrow, Archibald W. Black, William C. Harmon, and Joseph Mandula to the largescale, automated, production-line eddy

current test machines for tubes, bars,

and billets in use today. (Other steel

companies had early inventors and developers of electromagnetic tests but, in

many cases, their managements did not support their continuing developments

over a period long enough to achieve

practical applications.) Within the

General Electric Company, an early se- quence of inventive development was pioneered by men like James A. Sams,

Charles D. Moriarity, and H. D. Roop. Ross Gunn of the U. S. Naval Research

Laboratory pioneered a new form of probecoil magnetizing system with two small

diameter pickup coils displaced symmetrically along a diameter of the magnetizing coil. This was an early example

of use of one size of coil for magnetization, and of pickup coils of much

different size, in non-concentric positions. (See Tables I and II for details

of operation of these test systems, and

for other examples from this period.)

10
14.2 Post World War II developments

in electromagnetic induction tests

Rapid technological developments

prior to and during World War II (1941-

1945) in many fields contributed both to the demand for nondestructive tests and

to the development of advanced test

methods. Radar and sonar systems made

acceptable the viewing of test data as images on the screens of cathode-ray

tubes or oscilloscopes. Developments in

electronic instrumentation, and in

magnetic sensors used both for degaussing ships and for actuating magnetic

mines, brought a resurgence of activity.

After the war ended, developments such as Professor Floyd Firestone's "supersonic

ref lectoscope" for ultrasonic testing,

and Dr. Freidrich Forster' s advanced eddy

current and magnetometer systems, became

available as industrial nondestructive

testing systems. These systems offered

new dimensions for nondestructive mea- surement both of material properties and

of discontinuity locations and relative

sizes. The ten-year lag (from 1945 to about 1955) in industrial management's

acceptance of novel developments was uniquely short, in the case of these

instruments. Electronic instrumentation

based upon vacuum and gas-filled electron

tubes was approaching the peak of its

development. These developments permitted easy construction of variablefrequency oscillators and power supplies

for the magnetizing coils of eddy current

test systems. They also permitted minute

voltage or current signals to be amplified linearly to levels adequate for

display systems, graphic and permanent

recording systems, and for operation of

sorting gates, automation of scanning,

and mechanization of materials handling

during tests. Aerospace and nuclear

power industries were developing rapidly,

and made unique demands for sensitivity

and reliability of instruments for ma- terials evaluation and reliability as- surance during service. These industries

(and government agencies related to these

industries) were the primary sponsors of

research to advance the art of all forms

of nondestructive testing. However, in

the case of eddy current instrumentation,

governmental support was significantly

less than in other fields of nondestructive testing, for two reasons which are discussed next.

15. Development of Quantitative Eddy

Current Test Systems By Institut

Dr. Forster

By far the most important factor

contributing to the rapid development and

industrial acceptance of electromagnetic

induction and eddy current tests during

the 1950-1965 period in the United States

was the introduction of sophisticated,

stable, quantitative test equipment, and

of practical methods for analysis of

quantitative test signals on the complex

plane, by Dr. Friedrich Forster. Dr. Forster is rightly identified as the

'father of modern eddy current testing.'

His experience prior to World War II included advanced university education in

physics and a significant introduction to electromagnetic measurements related to the metallurgy and structure of steels

and non-ferrous metals in German research

institutes. During World War II, this

advanced knowledge was used in naval

warfare, particularly with respect to magnetic mines. At the conclusion of the

war, after a period of imprisonment by

the French, Dr. Forster retrieved his

technical reports and, "with the aid of a screwdriver and a technician," began his

further development of electromagnetic

test instruments in the upper story of an old inn just a few miles from Reutlingen,

where he later established his Institut

Dr. Forster. By 1950, he had developed

precise theory for many basic types of

eddy current tests- , including both absolute and differential or comparator test

systems, and probe or fork coil systems

used with thin sheets and extended surfaces. Painstaking calibration tests were made with these coil systems and

with mercury models (in which defects

could be simulated by insertion of small

pieces of insulators). Each test was confirmed also by precise solution of

Maxwell's differential equations for the

various boundary conditions involved with

coils and test objects, at least for

symmetrical cases such as round bars,

tubes, and flat sheets where such mathematical integrations were feasible.

Further studies were made of the non- linear response characteristics of ferromagnetic test objects, and methods

utilizing very low test frequencies (5

Hz), harmonic signal analysis, comparators at various levels of magnetization, and precise bridge circuits were

developed. In most instances, Dr. Forster

replaced measurements of the inductance or

11
impedance of test magnetizing coils with

the more precise technique of measuring

response with unloaded 'secondary coils'

coupled to the test materials almost identically with the magnetizing coils. The

extent and depth of these scientific

studies were not matched by any laboratory

in the United States, whether under government sponsorship or operating independently. By extensive publications (not

initially in the form of U. S. Patents,

but in the open literature), Dr. Fb'rster

made the end results of this research

available to the world of technical personnel. His monumental contribution of

almost the entire theory and technology of

electromagnetic induction and eddy cur- rent test techniques to the ASNT Nondestructive Testing Handbook in the 1955-

1959 period provided the means for educating thousands of other nondestructive

test personnel in the theory, methods,

equipment, and interpretation of eddy cur- rent tests. This integrated presentation

was then used throughout the world to up- date eddy current test technology.

16. Importation of Dr. Fb'rster' s Eddy

Current Technology to the United States

The unique developments in Dr. Fb'rster 1 s new laboratory in Reutlingen,

West Germany, were made known in the

United States, not only by those capable

of reading his publications (in German)

prior to 1950, but also by missions in

which American personnel were sent to Dr. Forster's laboratory for education and

experience with these new forms of test

instrumentation. Richard Hochschild, for

example, made a visit of perhaps six

months in Reutlingen. Upon his return, he

prepared summary reports which were distributed by the AEC sponsors of his visit.

Other personnel from private industry and

from other laboratories made visits to learn of these new techniques. In the

United States, numerous facilities began

research to test these new concepts and

instrumentation, including significant

efforts at Oak Ridge National

Laboratories, at the Hanford Works, and in

other facilities. The splendid creative

work of Mr. Hugo L. Libby at Hanford

during the past quarter century, and that

of Robert Oliver, Robert McClung, Caius V. Dodd, J. A. Deeds, and others at Oak

Ridge, which have continued into the

1970' s, may well have initially been

inspired (and sponsored in response to)

the new work done by Dr. Fb'rster.

However, even more significant has

been the complete transfer of Dr. Forster's advanced technology to enterprising American firms manufacturing and

distributing nondestructive testing

equipment, since 1952. As many of you may

remember, Dr. Forster made his first

presentation before an ASNT audience early

in the 1950's, after learning aboard ship

about five words of English, namely:

"Sonny boy" and "I love you." This first

personal presentation in the United States

was followed by meetings with management

of the Magnaflux Corporation, in which the

author served as a technical advisor to explain Dr. Forster's designs and

discussion. Agreements for licensing

under Forster patents were later

concluded, and the basic Forster

instruments were "Americanized" by use of

U.S. components and electron tubes, for

Magnaflux, by the NDT staff at Battel le

Memorial Institute in Columbus, Ohio.

(Here, again, the author had an opportunity to become aware of the

remarkable character of these new instruments.) During the next few years, increasing amounts of technology were transferred to Magnaflux, whose staff

(under Dr. Glenn L. McClurg) became qualified in the design and production of Dr. Forster's various instruments, and then

marketed these electromagnetic induction

test systems throughout the United

States.

17. Proliferation of Sources of Eddy

Current Equipment Derived from

Dr. Forster

The collaboration between Dr. Forster and the Magnaflux Corporation

lasted perhaps ten years, during which

rapid progress was made in both the German

laboratory and in the United States in

advancing the art of eddy current testing.

Upon completion of the arrangement with

Magnaflux, Dr. Forster marketed his instruments through the Forster-Hoover

organization in Ann Arbor, Michigan. Rudy

Hentschel , who was trained in Reutlingen

at Institut Dr. Forster, provided information transfer to this new organization.

(More recently, he has developed similar

advanced instruments at his own facility

in Ann Arbor.) After a few years, the

licensing of Forster instruments to Automation Industries, Inc. resulted in further transfer to advanced technology, and

marketing of equipment throughout a new organization. The most recent arrangement

with Krautkramer-Branson has repeated this

12
unique educational process. At present,

the large organizations manufacturing

many types of nondestructive testing

equipment and marketing their services

widely in the United States are presenting

updated versions of Dr. Forster' s basic

test instruments and modifications developed by their own staffs. Also in the

market are the instruments developed by

Magnetic Analysis Corporation, those based

upon Hugo Libby's research at Hanford (by

Nortec), those based upon the Oak Ridge

Laboratory research and developments by

Richard Hochschild and Donald Erdman

(which have migrated from the originators

through the Budd Company, Automation

Industries, and Tech-Tran in recent

years). Basically, in 1977, these various

brands of conventional eddy current

instruments are redundant and similar in

nature, having been updated to semiconductor circuit elements and more recently to integrated circuits in some

cases. With the typical instruments used

to cover various needs and applications,

the presently-available instruments

operate with absolute or differential

probe coils, encircling coils, internal

bobbin coils, and various special coil and

circuit arrangements, many of which were

described in the 1959 ASNT Nondestructive

Testing Handbook by Dr. Forster. Selfbalancing or adjusting instruments,

which establish reference points simply by

the placing of probes upon reference test

materials or specimens, are available in

several cases, utilizing developments by

Hugo Libby and other innovators. Designs

of probes based upon digital computer

analyses of eddy current distributions in

single- or multiple-layer sheet materials

have been made feasible through the

pioneering work at the Oak Ridge National

Laboratory. Special probes with split

coils, internal magnetic shields, and

other complexities have also been

developed for crack detection and other

special applications. Digital displays of

test signals are also being introduced.

18. Introduction of Microwave

Nondestructive Test and Measuring Systems

At very high frequencies, electromagnetic fields can be concentrated into

beams and propagated through space. When

such a beam pulse strikes a conducting

metallic surface, for example, it is

reflected and may return as an echo to the

site of the original pulse transmitter, or

to other detectors, as in radar detection.

In dielectric materials, microwaves can

be subject to rotations and phase shifts,

as well as to attenuation due to dielectric hysteresis losses. In many ways,

microwave nondestructive test systems are analogous in performance applications to immersion ultrasonic test systems. By

Maxwell's theory of the electromagnetic

field, microwaves are reflected like light waves by eddy currents induced in the

surface layers of highly-conducting metallic materials. Thus, microwaves appear to have the capacity to apply high-frequency

eddy current tests to a metallic surface

from a distance, and perhaps to scan such

surfaces to detect discontinuities which

change the pulse-reflection patterns.

When the Radac eddy current systems were sold to the Budd Company, Richard

Hochschild turned his attention to formation and development of Microwave

Instruments Company in Corona del Mar,

California. Soon a series of instrument

systems had been developed, and the long

task of educating industrial and scientific users in the capabilities and

applications of electromagnetic tests had

to be done all over again for these new higher frequencies. Of course, the theory

and design of microwave generators, horns,

antennas, detectors, and display systems

had been previously developed for

long-distance ranging in radar. Many

textbooks presented the electromagnetic

theory of microwaves in terms readily used

by electrical engineers. Microwave system

components and electron tubes were commer- cially available. However, these electrical engineers rarely were aware of the

needs of nondestructive test engineers,

and NDT engineers had little familiarity

with microwaves. In fact, many NDT personnel were still struggling to catch up with the art of eddy current testing at the lower frequencies, as explained by Dr. Forster. After several years of diligent

development and continued application

research and marketing efforts with the

assistance of Ron Botsco, Microwave

Instruments Company was sold and its

proprietor moved to greener pastures in

the area of medical services. A few other

organizations built simple microwave test

systems, but the development of industrial

microwave nondestructive testing has been

languishing during the 1970' s. Limited

research sponsored by ARPA and other

government agencies has resulted in indications of possibilities of crack

detection from a distance, since slots and

wires simulating discontinuities in metallic test object surfaces can be detected

under proper conditions of microwave

13
pulse-reflection testing. (The theory of

microwave antennas and of time-domain re- flectometry of microwaves in tubes, passing along wires, and reflecting and refracting in dielectric layers, offer many indications of potentially-valuable nondestructive test applications.) Since mi- crowaves can be focused, microwave systems could also potentially be designed

analogous to optical instruments and test

systems, as well as ultrasonic test systems. However, in 1978, there appears to

be no significant commercial development

or application of microwave nondestructive

tests in progress.

On the other hand, a large-scale ex- ample of microwave exploration of objects

at great distances is occurring in radio

astronomy laboratories throughout the

world. For example, Professor John D. Kraus of The Ohio State University has

constructed a large radio telescope in

Delaware, Ohio, and is using it continuously to map the universe of radio stars

and objects which emit microwave signals.

The mapping has progressed to where many

radio sources found have been confirmed by

films from optical telescopes, and others

have been predicted in location. Possibilities of emissions from galaxies,

'black holes', and other astronomical

features still exist. Professor Kraus has

recognized this as a form of "nondestructive testing of outer space" and has

written a delightful biographical book

"The Big Ear," which summarizes a lifetime

of study and applications of Maxwell's

theory of electromagnetic fields in clear

and simple words.

19. Advantages of Eddy Current Test

Systems Commercially Available in 1977-78

The eddy current test systems available commercially in 1978 have many advantages which justify their present wide

usage. One great advantage is the reproducibility of measurements possible with

many well-built instruments and test systems. Absolute conductivity meters and

instruments designed for thickness mea- surements of specific metals and alloys

are often quantitative and can have accu- racies of 1% or better. Comparison instruments permit unique sensitivities for

detection of discontinuities and of variations in material geometries or properties. With stable reference specimens,

tests can also be repeated with a high

degree of confidence. Instruments with

phase and amplitude signal capabilities

which duplicate phase-plane data consistently permit a wide range of interpretations to be made, depending upon the

strategic test conditions selected. Phase

separation of signals to suppress unwanted

signals and provide desired signals without interfering effects are especially

j valuable where consistency of geometry

and physical properties of test materials

permit their use. The general use of ref- I erence standards with drilled holes, milled ! or EDM slots, stepped wall thicknesses,

and certain natural defects provides a quick means of assuring proper operation i during testing, or of calibration and adjustment of control settings at the begin- ' ning of test sequences on objects of a par- i ticular type or material. These advantages ' generally accrue with nonferromagnetic test I materials and symmetrical simple shapes of

test objects. They cannot always be at- tained with magnetizable test materials or with parts of complex geometry where re- producible positioning may not be feasible.

By use of magnetic bias (or 'satura- tion magnetization'), depths of penetration of eddy currents and a.c. magnetic

fields into ferromagnetic materials can be

greatly increased. Many simple detectors

of surface discontinuities operate quite j effectively during automatic scanning

despite difficulties due to surface roughness or varia ions in hardness or magnetic

permeabilities in test objects. Largescale through-coil test systems for

smaller-diameter rounds and tubes, and ^ orbiting probe coil systems used with I rods, welded tubes, and even rectangular

j billets have been developed to a high

degree of ruggedness, serviceability, and

reliability for use in steel mills and on i large-volume inspection applications.

Automatic marking of defect locations ! permits salvage by grinding out and

welding repair (if the latter is needed)

on production line operations. Detection

of defects in surface layers of steels is

well-developed, but measurement of physical or metallurgical properties of steels

is generally not feasible by eddy current j, tests in the United States. One basic | source of difficulty is the sequential use of sheets, tubes, or rounds from different

mills or different heats, in rapid succes- j sion on production lines. Although the

chemical and physical properties of these i

steels from different sources may meet

manufacturing requirements adequately, no effort is made to control the magnetic ,1 permeability properties of these steels to

j any type of calibrated standard. As a I

14
consequence, random variations in magnetic

permeability prohibit the development of

reproducible correlations between absolute

measurements of eddy current test signals

and the actual physical or metallurgical

structures of the test objects.

High sensitivity to material electrical conductivity (in nonferromagnetic ma- terials) has been attained with small

probe coil test instruments typically operating in the range of 64 kHz test frequencies. Such small coil probes tend to

be sensitive to lift-off, and 'lift-off

compensation systems' such as those developed by Dr. Fb'rster are often used to correct lift-off effects over a small

range. Similarly, small differential coil

or field detector systems provide high

sensitivity to surface cracks in both non- magnetic and in ferromagnetic materials.

However, in general, for such crack detection, manual positioning and scanning with

these fine probes is usually required on nonsymmetrical part surfaces or materials

in service structures and machines. There

is no low-cost means for total inspection

for cracks on parts with complex surfaces,

such as those for which liquid penetrant

tests (or magnetic particle tests on iron

or steel parts) provide overall surface

inspection at high speed and low costs.

20. Limitations and Disadvantages of

Presently-Available Eddy Current Test

Systems

The primary disadvantage of eddy current test systems available in 1977-78 is

the fact that their test indications are psychologically-unacceptable. They are

far less effective in stimulating man- agement and worker comprehension and

corrective action than the graphic images

provided by other processes such as liquidpenetrant, magnetic-particle, or x-ray

inspection, for example. These eddy cur- rent tests fail to produce a clear, visible, interpretable image of defects or discontinuities from which an almostinstant recognition of their nature,

shape, size, or location is obvious to all

observers. Thus, where the purpose of

nondestructive testing is to motivate

personnel to best efforts or to permit

immediate correction or repair of defects,

eddy current tests which produce fugitive

traces on cathode-ray tube screens or 'meaningless' movements of the needle of a panel instrument, are quite ineffective.

Secondly, even when these tests are used

to measure material properties or dimensions, the fact that the quantitative

displays of signal amplitudes, component

values, or phase angles have no direct

meaning to the untrained observer acts to create doubt. When numbers have to be

'looked up in a book or chart' to find the

real meaning of test indications, the opportunity exists for human errors. In addition, since the same book or chart

would not be valid for materials other

than a specific material for which the

chart is designed, untrained observers

will question the results. If modern eddy

current tests provided clear, informative

images or direct read-outs in numbers of a specific dimension, property, or service

characteristic (which could be immediately

checked on reference samples if needed),

their use could be multiplied indefinitely. For example, where today x-ray or ultrasonic tests are specified for control

of weldments, no one dares to trust eddy

current measurements of these same welds

for control of welding operators or for

acceptance of the welds for specific

service conditions.

The second disadvantage of present

eddy current test systems is that they are greatly limited by artificial constraints

inherent in the thinking of present designers, manufacturers, and users of these

tests. No one has made any fundamental

change from the basic designs which Dr. Forster provided in . 1955, nor in the

methods for interpreting test signals.

Because of these unnecessary constraints

adopted by tradition, eddy current tests

are far less informative or sensitive than

they should be. Examples of such mental

straight- jackets are cited in a succeeding

paragraph. True advancement to the next

era of eddy current testing cannot occur

until the responsible and active engineers, management, and test personnel

develop systems to utilize the full capabilities of the method and use these

systems for effective control of people,

processes, products, and in-service ma- terials and systems.

The third disadvantage of present

eddy current test systems is that they are limited in penetration depths (often to less than 5 or 10 mm) and in magnetizing

coil and detector adaptability to rough or contoured test material surfaces. Few

probes or test coils are designed to fit

into a sharp inside corner or intimately

to the outer edge of a sheet material, for

example. In general, many probe coils are

on rigid forms, and cannot conform to

15
irregular contours on test objects. It is

also often assumed that smal 1 -diameter

probe coils must be used to measure fine

defects or the properties of small areas

of test objects. Yet, small coils assure

lack of deep penetration of the magnetic

field into metals or alloys (since the

coil field in air is proportionately

smal 1 ).

The fourth disadvantage of present

eddy current test systems is their insensitivity to local conditions or discontinuities which produce only small distortions in eddy current flow paths. (In

this sense, "small" is related to the

diameter of the test coil.) In general,

present test systems do not detect discontinuities or defects which lie outside

the perimeter of the test coils. They are also typically insensitive to small defects which lie on the centerline of the

test coils. In fact, existing test coils

integrate all magnetic flux lines which

their winding turns enclose. With discontinuities small in dimensions compared to the coil diameter, the defect signals are submerged in a large average coil signal

so that highly-sensitive detector circuits

are needed to detect the minute changes in

amplitude or phase. Even worse, present

coil -type detectors are insensitive to the

tilt or angle of magnetic flux lines encircled by the coils. They simply measure

the time rate of change of the total magnetic flux enclosed by the test coil.

This often loses signal magnitude by

ratios as great as 100 to 1. Finally, the

use of coils for detection of signals

limits the most minute area detectable to that roughly corresponding to the pick-up

coil diameter (or the diameter of a ferromagnetic core within the pick-up coil.)

Modern microelectronics can far exceed

these limitations on reducing the size of

test area whose electromagnetic test

signal is detected and displayed.

The fifth disadvantage of present

eddy current test systems is their limitation to higher test frequencies and to tests at larger phase angles on the com- plex plane. The voltage signal amplitude

provided by a pickup coil is proportional

to the test frequency. If an effort is made to operate at very low test frequencies to attain deeper penetration and re- sponse to 'rear-surface' conditions, the

signal can become too low to detect in the

presence of normal noise signals. Even if

amplification can permit signal display,

the low-frequency test condition leads to signal points on the upper left portion of

the locus curves of response on the com- plex plane. Here, the signal closely

approaches the 'empty-coil signal' in both

amplitude and phase. The small contribution of the eddy current losses to this

test signal also imply lack of test

sensitivity.

The sixth disadvantage of some present eddy current test systems is the

variation of magnetizing current amplitude

with test frequency. Higher test frequencies require higher power supply voltages to provide a given magnitude of cur- rent in the test coils. If variations in

test frequency result in inverse changes

in magnetizing current, tests may be made

on ferromagnetic test parts at widelydifferent levels of maximum magnetization,

at different test frequencies. This can create difficulties with harmonic signal

generation and non-linear response characteristics in eddy current test measurements. Alternatively, if true constant

-current magnetization levels cannot be

provided as frequency varies over a wide

range, the designer may limit the test

instrument to one or a few discrete test

frequencies for which constant current

levels can be assured. Even when multifrequency tests are made at these few

frequencies, a loss of information at other intermediate frequencies results.

A final limitation of some present

eddy current test systems is their use of

sinusoidal continuous ac current excitations. A useful signal thus lost is

that of magnetic retentivity, and its

relation to eddy current pulse decay characteristics. Square wave or spike excitation can provide both retentivity signals

and decay curves for eddy currents within

the test materials. The use of coil -type

pickups prevents detection of the dc

components of test signals which could be

generated with pulse or rectangular

waveshapes, since response is zero to steady- state magnetic flux conditions.

21. Artificial Constraints in Design and

Use of Eddy Current Test Systems

Possible present stagnation in development of new or unique forms of eddy

current test systems could result from

constraints in thinking about novel ap- proaches, perhaps because these new con- cepts are not fully documented in the past

history of eddy current inspection. For example, circular test coils were selected

for initial investigations because they

16
were easy to build and many test objects

had circular symmetry. Theory also has

been directed to circular test coils since

the solutions of Maxwell's equations for

the electromagnetic field could be at- tained more easily with symmetric circular

boundary conditions (such as can be solved

with Bessel's equation and its modifications). Actually, however, test coils

can be wound around square, triangular, or spherical forms. They could be made

highly flexible so that they can be made

to conform to surfaces of any shape. In all cases, advantages accrue in eddy cur- rent testing if the magnetizing coil liftoff can be minimized. Flexible magnetizing

coils with stranded conductors imbedded in

rubber- like sheets or tubes might offer

considerable advantages. Applied under

pneumatic or other pressure, such flexible

sheet magnetizing coils could be fitted to gently curved test parts with essentially

zero lift-off. If the detector coil could

also be in intimate contact with the

curved surface of the test object, maximum

test sensitivity and elimination of non- uniform lift-off conditions could be

attained.

A further typical constraint lies in

the assumption that the magnetizing coil

and the pick-up coil should be either (a) one and the same coil, or (2) of identical

diameter and coincident in position.

True, the literature describes such simple

arrangements redundantly. However, the

pickup coil could be of any diameter

(preferably smaller than the magnetizing

coil), and be placed at any angle and in

any desired position with respect to the

magnetizing coil. For example, the pickup coil could be located at any point, and

in any orientation, within or completely

outside the annul us of the magnetizing

coil, or even at a point directly under

only one point of the magnetizing coil

winding. In fact, if the pickup coil is

replaced by a semiconductor magnetic field

detector, total freedom exists with

respect to the number, positions, and

angulations selected for the individual

semiconductor detector elements. For

example, an array of semiconductor detectors could be placed anywhere within,

under, or external to the magnetizing coil

windings to provide a multiplicity of

input signals with only one magnetizing

coil. Ideally, such an array should cover

the entire area enclosed within the magnetizing coil or be extended over an area

much larger than the magnetizing coil to provide total test information concerning

the entire eddy current test field created

by the magnetizing coil. If this detector

array could be interrogated in sequence by

rapid techniques such as used to read

computer memories or to digital ize images,

for example, the resultant multi -channel

data could be analyzed by digital

techniques, and displayed in any desired

image format (including two- or threedimensional images on a television

screen).

A particularly desirable change from

prior art would be to utilize very large

diameter magnetizing coils closely fitting

test-object contours, to assure deep geometrical penetration of the magnetizing

field. For example, a 10 in diameter test

coil could easily project strong magnetic

fields 2 or 3 inches in front of the coil

face. Used with lower test frequencies,

such a coil might provide penetration

through 1 or 2 inches of nonmagnetic test

material (particularly in the case of ma- terials with electrical conductivities

less than about 10% IACS). With arrays of

semiconductor type magnetic field detectors, detail sensitivity to near-surface

discontinuities might become sufficient to provide good recognizable images of typical discontinuities and defects. Alternatively, a linear array of magnetic field

detectors might scan linearly across the

field, or be rotated to provide a circular

scan of the field within or adjacent to the magnetizing coil. The instantaneous

appearance of a recognizable eddy current

image of defects would convert this test

into a psychologically-acceptable test and

greatly increase demand and use for eddy

current tests. The repeatability of such

images, as coil and probes are moved over

test surfaces, or tests are repeated after

a time period, would do much to establish

confidence in the reliability of such

images. In general, the instantaneous

character of eddy current images and the ease with which depth sensitivity could be

changed or polarized eddy current flow

established, might compare favorably with

x-ray or ultrasonic test images of welds

or with fluorescent penetrant or magnetic

particle tests of surface cracks or seams

and laps.

Another potentially attractive technique is that of using differential probe

signal pick-ups (preferably by detecting

unbalance in a four-detector array analogous to a Wheatstone bridge) which would

be a direct map of the flow of eddy cur- rents below test surfaces. The reality of

eddy current flow paths and their deviations caused by discontinuities could then

17
be visualized readily. Since local detectors in the vicinity of crack ends, for

example, can have surprisingly large test

signals (as compared to those of large- area pick-up coils), unique opportunities

exist for precise measurements of crack

lengths and of crack extension rates. These topics are of special interest where

fracture mechanics analyses are to be made

of cracks to determine their capability to propagate under service stresses.

22. The Pending Revolution in

Microprocessor Control of Eddy Current

Tests

Already upon us in the 1977-78 time

period is the explosion of use of microprocessors and digital computer techniques

as integral components of nondestructive

test systems and controls. The costs of

these components have become so low that

they are now toys for amateurs 1 i ke the

older "ham" radio operators. Home computers are available in the corner computer

store which rival large-scale digital

computers of just a few years ago. Microprocessors have already invaded control of

ignition and carburetors in automobiles or domestic appliances in the house-wives'

kitchens. They are urgently needed in

eddy current instruments, where EPROMs

(erasable read-only program memories) can

be dedicated to specific purposes, such as providing direct correlations of eddy cur- rent test signals with material dimensions

or properties. Since small test instruments could now be made direct reading for

any valid measurements by eddy currents,

the old business of table look-up or intelligent interpretation has become obsolete. The advantages are obvious in that

each purpose could involve a separate lowcost test instrument, or plug-in PROM's

could be used to change the correlation

data from one test material to another or from one test frequency to another.

The additional advantages to be at- tained by integration of microprocessors

and computers into eddy current test instrumentation are the possibilities for

much more sophisticated real-time analysis

of test signals. Positions of signal

points could be determined on the complex

planes, the directions of signal change

established in response to each test material variable, and undesired signal com- ponents could be eliminated, without the

use of the analogue circuits used previously for such purposes. In addition,

incoming test data could be continuously compared with prior data (from the same or other test objects) to detect and

define differences resulting from discontinuities or changes in material properties. A further operation of datasmoothing as point-by-point data are entered into memory could add an additional degree of precision. Simple extrapolation or interpolation estimates could

be derived from test data so that changes

in trends could be detected rapidly as tests progress. Of course, differential

measurements, or comparison measurements,

could be made also from absolute input

signals, thus eliminating the need for

several test arrangements (absolute, differential, or comparison coils) to attain

full information from eddy current tests.

A further natural consequence of use of digital techniques in data collection

and analysis would be the possibilities

for real-time control systems based upon

eddy current test inputs. Recognition of

material damage in service, high stress

levels, or high temperature effects could

be used to shut down or control systems to prevent premature failures. In addition,

in processes such as fusion welding, a multiplicity of eddy current detectors

could be used to monitor and control the

welding process. Input signals might be

derived from changing conditions such as material thickness, edge or weld groove

distance, conductivity of metal, temperature of metal , depth of penetration of

fusion, and final weld inspection for root defects, undercutting, cracking, lack of

penetration at the root, and other undesired conditions. Other similar production control applications could be

cited.

Still another advantage could be at- tained in telemetering and storage of eddy

current test data. Digital data could be

stored in computer memory, transferred to magnetic tapes, floppy discs, or other

large-scale memories, and used as permanent inspection records (much as data on nuclear pressure vessels obtained by ultrasonic tests are digitized and stored

today). In addition, digital data storage

may in the future permit direct correlation of conditions detected by one type of

test (such as ultrasonics) with another

type of test (such as eddy currents).

18
23. Future Development of Direct-Imaging

Eddy Current Test Systems

As noted earlier, the greatest limitation of eddy current tests, as compared

with more popular tests like x-ray, penetrant, magnetic particle, and C-scan ultrasonic tests, is the lack of interpretable images derived from eddy current

tests. Of course, there is no reason why

eddy current test probes could not be

scanned over test-object surfaces, just as

is now done in immersion ultrasonics to establish C-scan (or plan view) images

showing defect locations on the test surfaces. However, such scanning is slow and

costly which inhibits its use even with

ultrasonic testing. If eddy current tests

could provide instant images (somewhat

like x-ray fluoroscopy) or permit re- cording of test information so that it

could be displayed on the face of a cathode-ray tube like a television picture, the data could become psychologically attractive and understandable to many more observers. In addition, if eddy

current test images could be informative

of conditions through much greater metal

thicknesses so as to compete effectively

with x-rays and ultrasonics, their usefulness could be increased.

With semiconductor detectors, such as indium-arsenide Hall devices, the detector

size can be made quite small (as compared

to the diameters of large magnetizing

coils). If the semiconductor detectors

were formed into arrays (like checkerboards) of perhaps 100 by 100 elements or more, within a magnetizing coil of large

diameter, the individual picture elements

could then be read-out one-by-one in se- quence, just as microprocessor or digital

computer images are recorded and reproduced on the X,Y coordinates of a television picture tube screen. (The hobby

computers are now often equipped with

facilities and programs (in software or PROM's) for such data display as images in

color.) Such data could be collected from

the detector array, subjected to interpretation criteria, and the results displayed on a full -screen image in short

times, such as one or two seconds. They

could then be interpreted from the screen, particularly if the microprocessor also

presents needed digital data correlations

for test conditions and test-object dimensions and properties. Such tasks as determining if discontinuities were located

in critical areas of test parts could also

be carried out by the microprocessor if

critical areas had been identified and

mapped similarly for the specific test

objects in advance.

A further potential advantage of

television screen imaging of eddy current

test signals could be the low-cost, highspeed production of permanent video tape

records of all test conditions and

results. Such video tapes can be made

today on recorders so low in cost that

they too are becoming toys in the home.

Such video tapes are easily transported

and can be played back later and at other

locations for review of test results.

Video-taped images of standard reference

specimens and defects might also be used

during visual evaluation of the eddy current test images. Also feasible, in these

cases, is digital enhancement of image

contrast, and display of enhanced images

in various colors or with various

brightness levels which could be adjusted

for best discrimination of significant

discontinuities or defects.

24. Future Development of DeepPenetration Eddy Current Test Systems

The future should see a huge improvement in the depth capabilities of eddy

current test systems. At present, most

eddy current tests are used for surface

and near-surface inspection where they

provide high test sensitivities.

Uniquely-good performance can be attained

with thin-wall test objects, namely those

whose metal wall thickness is a small fraction of the eddy current penetration

depth. (Eddy current penetration depth is

the metal depth at which the eddy current

density, J, is reduced to about 37% of its

value at the test material surface closest

to the magnetizing coil.) At a depth of

three times this penetration depth, the

eddy current density is only about 5% of

the surface current density. At five

times the penetration depth, the eddy cur- rent density is negligible at less than

0.5% of its surface intensity. The stan- dard penetration depth is an inverse function of the square root of the product of

test frequency, material conductivity,

and/or material magnetic permeability. In highly- ferromagnetic test materials, the

penetration depths are typically reduced

by a factor of 10 to 100, as compared with

a nonmagnetic test material.

Improvements in penetration depth are obviously attainable by lowering the test

frequency or saturating ferromagnetic

materials to lower their relative magnetic

19
permeability. The first technique, of

lowering test frequency, was limited in

the past by the difficulty of detecting

low test frequencies with pickup signal

coils. With semiconductor detector systems, or by adding integrating operational

amplifiers to pickup coils so as to integrate the test signals, it should be possible to work at much lower test frequencies. If, as an example, it were

feasible to lower a conventional 10 kHz

eddy current test frequency to 1 Hz, the

standard penetration depth should increase

by 100 times. However, at such a low test

frequency, it would take one second to complete one cycle of alternation. With

modern electronic integration systems,

such frequencies are not out of the range

of feasible measurements. In fact, low

frequency oscillators and analysis systems

should be able to handle frequencies as low as 0.01 Hz.

However, increasing the penetration

depth by lowering of test frequency is of

no value if the magnetizing coil diameter

is such that the magnetizing field in air

is reduced geometrically so that very few

or no flux lines can reach the new penetration depth limits. The answer here is to employ large-diameter magnetizing coils

(although the eddy current detectors can

be as small as desired). An example of a large-diameter magnetizing coil in present

use is the metal detector used to inspect

air passengers prior to boarding aircraft

in the United States. Such large-size

test coils should also conform to surface

contours of test objects, where feasible,

and provide adequate levels of low-enough

test frequencies to meet inspection re- quirements. Ideally, where feasible, the

eddy current test should also result in

interpretable images with good psychological impact, so that they can influence

both management and workers to their best

efforts.

25. Future Development of Time-Domain

Ref lectometry Eddy Current Tests

Time-domain ref lectometry is a wellknown technique for detection of discontinuities in high-frequency electromagnetic field transmission lines, telephone

and telegraph lines, and by radar. Similar time-domain ref lectometry techniques

are used in ultrasonic nondestructive

testing, and particularly in immersion

testing. Short pulses of high-frequency

wave trains, or a single step or square wave pulse, can be used. It is also possible to utilize short electromagnetic

pulses in through-transmission electromagnetic testing. For example, Paul Gant

of Shell Development Laboratories in

Emoryville, California, used such a system

many years ago to transmit electromagnetic

pulses along oil well drill pipe and steel

tubes. Encircling coils used as transmitters and receivers permitted detection

of larger discontinuities and of zones of

reduced wall thickness. Richard Hochschild

also used radar- type echo ranging with his

microwave test equipment to establish

distances to metallic sheet and other

reflectors.

Time domain ref lectometry and standing-wave analyses are widely used in highfrequency electronic engineering analyses.

Microwave parts and 'plumbing' fixtures

are available from electronic equipment

manufacturers, for construction of such

systems. TDR plug- in hardware is available for high-quality cathode-ray oscilloscopes, which can be used directly for

time-domain reflection eddy current tests.

In such tests, microwave pulses are transmitted along metallic or dielectric rods,

tubes, or sheets. Where impedance mismatch conditions are encountered, reflections occur. These systems are entirely

analogous to ultrasonic pulse-reflection

tests. Where the electromagnetic waves

travel in dielectrics or in air around a metallic conductor, reflection can result

from liquid or solid dielectrics (such as ceramics), or from metal surfaces (which

typically act as total reflectors). Such

techniques might apply for rapid inspection of metallic material moving at high

speeds in a rolling mill, or perhaps for

insepction of dielectric coatings being

applied to wires, tubes, or sheets under

fast transport conditions. (The travel

speed of waves encountered in typical

electromagnetic time domain ref lectometry

on metallic structures is perhaps two- thirds the speed of light (or about 2 x

108 meters per second.) Thus, echoes

would return from a reflector one meter

from the source in a time period of 10"8 seconds or 10 nanoseconds. Precision,

fast-response, high-resolution electronic

signal detection and analysis equipment,

such as a cathode-ray oscilloscope or digital systems, would be needed in most

cases (except when standing wave resonance conditions are present).

Further development of the recent

efforts to use microwave beams to interrogate metals surfaces at a distance, to detect conditions such as slots or cracks,

20
highly-conducting surface coatings, dielectric surface coatings, or projections

and surface irregularities, is still

needed to permit practical test systems to

be developed. In a similar sense, use of

microwave distance measuring devices to

detect movements of structures such as large tanks or bridges during earthquakes

or under service loading might also be

feasible. The still higher frequency

laser beams used to range distances in

surveying are similar, since their electromagnetic waves are still shorter in wavelengths than microwaves. As optical waveguides and signal transmission systems

improve, it may be possible that these

will also be used for analysis of electrical and magnetic properties of materials, and so join the ranks of practical

nondestructive test systems.

26. The Ultimate Goal: Intelligent

Materials with Microwave Trouble Signals

The ultimate goal with all forms of

nondestructive test system development

should be that of discovering or developing 'intelligent engineering materials'

which detect troubles by themselves and

transmit suitable alarm signals in time to

permit human control to prevent disastrous

failures. The presently-available technique of acoustic emission nondestructive

testing is an example of transmission of

signals from materials under mechanical

stresses or subject to damaging events

such as stress corrosion or fatique damage

leading to cracking. Man has not tried

very hard to hear the many signals emitted

by natural and artificial materials. Re- cent interest has been directed to earthquake prediction and prediction of dangerous storms. Probably many nondestructive

test engineers have not bothered to "listen" to the microwave signals emitted by

metallic surfaces and structures under

stress, vibration, or surface attack. Yet,

engineers often have to work very hard to muffle or destroy these signals when they

tend to interfere with intentional human

microwave or radio transmissions. For

example, it has long been known that railway axles rotating in journal bearings

create radio "noise" which is considered

objectionable. In fact, copper straps are applied to short-circuit these emissions

to assure that they do not interfere with

railway signal systems or other communications.

Every citizen who drives a car with a radio has also had an opportunity to observe the microwave signals from large

trucks, bridges, and machinery. If the car radio is tuned between broadcasting

stations, so as to receive only "static"

noise signals, variations in these signals

can be quickly found as his car passes

large trucks with metallic bodies, or drives across older iron or steel bridges

with loose bolts or connections. If longdistance static is screened out (as in a shielded room), the radio signals from

contacts dragging across metal surfaces,

or from rotating bearings, or from loosely-bolted joints undergoing vibration, can

be heard distinctly. In fact, if while

wearing gloves, one taps a knife and fork

together while walking about in the vicinity of the radio receiver, he can send

Morse code or any other sequence of signals which can be heard on the radio loudspeaker. When two metals rub together, enormous sounds and screeches can be heard

as the metals complain of the damage their

surfaces are undergoing. When ball or roller bearings rotate under heavy load or with inadequate lubrication, each

metal -to-metal contact can be announced by

clicks and distinct signals. Often the same sequence of signals is broadcast with

each rotation of the shaft or of a ball or roller with a damaged surface. In all cases, the intensity of these signals can

be greatly increased by connecting one of

the metal surfaces to the antenna lead of

the radio (preferably through a shielded

cable). The other metal surface may be

grounded or allowed to stand insulated

from all other surfaces. On the other

hand, short-circuiting the two pieces of

metal together at the point of signal

generation generally extinguishes the

radio signals broadcast.

The well-known triboelectric effect

(electrification by friction) known by the

Greeks 2000 years ago, illustrates the

basis of such microwave emissions. During

rubbing, one material steals electrons

from the other material, particularly when

contact is broken. Since the electron

cloud within conducting metals constitutes

a plasma, the sudden removal or injection

of charge locally may create plasma oscillations. If one of the metal pieces is

insulated from the other, it is possible

that such oscillations result in electromagnetic waves traveling through the

metal. It then serves as an antenna to broadcast these waves into the space

around it. These weak signals can be

easily lost in static conditions. Tests

in a shielded room permit their clear

identification and their correlation with
material surface characteristics. The

author has found these signals to approximate 'white noise 1 , in that they can be

detected at all frequencies from those of

audio amplifiers, through those of a.m. and f.m. radio broadcasting, to frequencies of 100 MHz or higher. This could be

expected from the short time duration

involved in the robbery of electrons from

a metal surface.

Thus, the ultimate in-service monitor

system for metallic systems and machines

may well be formed of microwave monitors

of electron emissions. When the electron

charges are removed from a metal , the eddy

current reaction is one of high frequencies, capable of being transmitted through

the metal antenna and from it to detectors

at moderate distances. Increased stressing or rubbing of contacts across contaminated (oxidized) metal surfaces results

in enhanced microwave distress signals.

These same signals can be used to create

television images of metal surfaces, including geometric features such as scratches, or chemical features such as corrosion, oxides, contaminants such as fingerprints, or even the effects of adsorbed gas layers or amorphous coatings.

With a low-voltage electron beam scanning

such surfaces (as in a vidicon television

camera tube), the surface features are not damaged, and their images can be reproduced faithfully over long periods of

time. In this special case, conditions

reflected by eddy current reactions as electrons transit metal surfaces can be

imaged with remarkable clarity. Typical

images enlarged 30X show detail approaching a few micrometers in dimensions.

Appendix I

Electromagnetic Induction Non-Destructive

Tests

Principle of Operation:

Electromagnetic induction non- destructive tests are characterized by the

induction of varying electrical currents

in the test object by means of repeated

variations in an electromagnetic field.

This method contrasts with the electric

current conduction tests in which current

flows into the test object through direct

electrical contacts from an external

source. No input contacts are required

with induction- type tests. The induced

current in the test object produces

differences in electrical potential, magnetic fields, and heat or temperature gradients. When alternating or varying cur- rents are induced in ferromagnetic

materials, heat is produced not only by

ohmic losses proportional to the square of

the current density, but also by hysteresis losses in the magnetic material. The

total "iron" losses, composed of both eddy

current losses and hysteresis losses, are sometimes employed to indicate material

properties. The pick-up may detect variations in electrical potential distribution, in magnetic field strengths, in highfrequency electromagnetic wave properties,

in temperature, in mechanical force or torque, or in losses in the material of

the test object, or combinations of these

factors. A large number of patents cover

tests of these types (Tables III and IV).

Potential Pick-up Methods:

Electromagnetic induction tests with

potential contact pick-ups were proposed

for testing of lead-sheathed cables and

other tubular conductors by Atkinson (U.S.

Patent Reissue 21,853) and Edgar (U.S.

Patent 2,186,826). The method has not

been too popular, however, because it is

difficult to screen the pick-up circuit

from the exciting electromagnetic field

variations. Both Atkinson and Edgar proposed methods of introducing suitable neu- tralizing or compensating voltages in the

pick-up circuit, 180 deg. out of phase

with the disturbing emfs. Braddon (U.S.

Patent 2,074,739) provided a method of

locating the flaws in cable sheaths with a suitable indicator. Knerr (U.S. Patent

2,124,577) also included the use of potential pick-ups in his method for testing tubes and cylindrical objects.

Magnetic Field Distortion Pick-up Methods:

The detection of flaws by measurement

of distortion in electromagnetic fields

(which would have uniform intensity in the

absence of flaws) has found wide use. Chappuzeau and Emersleben (U.S. Patent

1,782,462) employed a series of coils

whose turns were parallel to the surface

of test objects, such as tubes, bars, and

rails, to detect deviations from normal

magnetic field distributions. They further employed tuned output and input circuits to obtain optimum response to harmonic signals. Stein (U.S. Patent

1,992,100) proposed tube- and bar-testing

apparatus which consists of main exciting

coils which set up a normally neutral zone

in the fields between them, and a test

22
coil located in the neutral zone, whose

output actuates an indicator such as a cathode-ray tube.

Several arrangements of exciting and

pick-up coils, including those in which

the exciting coil is placed within the

tube and the pick-up coils are placed outside the tube, were patented by David

(U.S. Patent 2,065,118). He also employed

electronic amplification of pick-up

signals, with provision for permanent

records or for marking the test object.

In U.S. Patent 2,065,119, he points out

that voltages induced by distortion of the

magnetic fields are exceedingly minute,

often as low as one-millionth of a volt.

Also, a variation of 0.0001 in. in the

position of the detector element from its

true electrical center in the exciting

field can produce comparable signals.

Furthermore, as the article being tested

moves through the exciting field, its mo- tion tends to deflect the electrical

center of the system in the direction of

its travel. Precision adjustments of coil

locations are essential to correct these

difficulties.

A simple means for testing interior

surfaces of tubes was proposed by

Greenslade (U.S. Patent 2,104,646), and

consisted of search coils connected in a Wheatstone bridge circuit.

Hay (U.S. Patent 2,150,922) produced

longitudinal a-c. magnetization in cylindrical test objects which were then ro- tated past a fixed pick-up coil to reveal

flaws. The depth of flaw penetration was estimated by varying the exciting frequency or by providing d-c. saturation of

ferromagnetic materials, so as to vary the

depth of penetration of eddy currents. A cathode-ray tube was used as an indicator.

In U.S. Patent 2,162,710, Gunn showed

a small probe containing exciting coils

and pick-up coils located so as to be sen- sitive only to distortions in eddy current

flow in the test object. The pick-up signal was synchronously rectified so that it

might actuate a sensitive d-c. galvanometer. An automobile tire nail detector

patented by Wages (U.S. Patent 2,502,626)

employed a vacuum tube amplifier and place

circuit meter to detect magnetic field

distortion.

A third harmonic in the pick-up signal was found responsive to flaws in test

objects excited uniformly with a 60-cycle

magnetic field, by Sams and Moriarty (U.S.

Patent 2,007,772).

Michel (U.S. Patent 2,489,920) used vacuum tube phase discriminator circuits

to actuate neon indicators and relays in a metal detector for use in manufacturing

linoleum. Bovey (U.S. Patent 2,495,627)

used a rectifier bridge to convert unbalanced field signals to a d-c. meter deflection in his metal -object sorter.

Transformer Pick-up Methods:

Several tests have been proposed in

which test objects form the cores of transformer arrangements. The primary coils

are excited with sinusoidal alternating

currents, and the secondary induced voltage magnitudes and wave shapes are examined to detect flaws or material properties. Kinsley (U.S. Patent 1,813,746)

proposed the use of a magnetic oscillograph to examine such wave shapes, as well

as the use of relays operating on the difference between secondary signals obtained

from standard and unknown test objects

such as tubes and bars. Bill stein (U.S.

Patent 1,958,079) proposed a rail tester

in which the secondary signal is amplified

and its magnitude suitably indicated. In

U.S. Patent 2,084,274, he claimed improved

sensitivity as a result of shunting the

exciting and pick-up coils with suitable

capacitors. Hallowell (U.S. Patent

2,010,189) employed a cathode-ray oscillograph with the exciting signal applied

to one set of deflection plates, and the

differential output signal (between standard and unknown test objects) applied to the second set of deflection plates. Ebel

(U.S. Patent 2,111,210) used concentric

exciting and pick-up coils of pancake

form, located in a plane parallel to the

surface of the cable sheaths under test.

A similar arrangement was patented by

Loewenstein (U.S. Patent 2,116,119), the

inner pancake coil diameter being designed

to intercept a component of flux greatly

dependent upon the thickness of the sheet

or tube being tested.

To detect small flaws by eddy current

flow in magnetic tubes and bars, Knerr

(U.S. Patent 2,124,577) rendered the material substantially nonmagnetic by subjecting it to a high degree of magnetic saturation with a saturating d-c. coil or strong permanent magnet. A transformertype exciting and pick-up circuit arrangement was used to compare standard and

unknown test objects. In U.S. Patent

2,124,579, he indicates that pick-up coils

should have small dimensions comparable to those of flaws to be detected, for optimum

sensitivity. A plurality of such small
coils, disposed over the surface of the

test object, may be required for complete

coverage.

Zuschlag arrives at similar conclusions in U.S. Patents 2,353,211 and

2,398,488, in which he proposes the use of

small pick-up coils near the surface of a rotating specimen, and suggests several

arrangements of circuit and detector.

Canfield (U.S. Patent 2,245,568) also proposes to detect flaws by detecting variations in eddy current flow, but he uses the quadrature component of pick-up flux

as a sensitive measure of changes in eddy

current resistance. This component gives

an indication which is relatively insensitive to changes in permeability of the

article being examined.

Irwin (U.S. Patent 2,290,330) developed equipment for the simultaneous independent measurement and recording of a magnitude related to the phase angle between excitation and pick-up waves, and a second magnitude characteristic of the

pick-up a-c. signal. Variation of leakage

flux, as through a shunt transformer path,

is employed by Thorne (U.S. Patent

2,311,715) to detect flaws which influence

the permeability of rails.

DeLanty (U.S. Patent 2,315,943) proposed to concentrate the flux in tubular

test objects by introducing low-resistance

inserts within the tube at the point of

testing. These high-conductivity inserts

have induced in them large eddy currents

which oppose the entry of magnetic flux

into the insert, and presumably con- centrate the flux in the tube wall under

test.

High-Frequency Electromagnetic Wave

Pick-up Methods:

Recent developments in ultra-high

frequency sources, wave guides, oscillators, and detectors, particularly in wartime radar developments, have contributed

new techniques to non-destructive testing.

Because of the normal time lag before issu- ance of patents, only a limited number of

such tests have been revealed. Two typical examples are given below:

Larrick (U.S. Patent 2,489,092) proposed the use of a high-frequency source

and open-ended wave guide against which

the surface of the test object is placed.

Surface resistance and the thickness of

nonconducting coatings are evaluated in

terms of the resonant frequency of the

wave-guide system.

Schlesman (U.S. Patent 2,491,418)

proposed that the standard and unknown

test objects be placed successively within

or across the opening of a high-frequency

cavity resonator. Changes in conditions

of cavity resonance would detect test objects differing from the standard.

Magnetic Loss Pick-up Methods:

Burrows (U.S. Patents 1,676,632 and

1,686,679) proposed the use of pick-up

coils adjacent to tubes and bars excited

by an alternating electromagnetic field.

The pick-up coils were connected to the

moving coils of a dynamometer relay or meter whose fixed coils were connected in

series with the exciting current, to obtain a measure of hysteresis losses in the

test specimen. The signal was obtained

from series-opposed pick-up coils, one

coil being used with a standard specimen,

and the second coil being used with the

unknown specimen. The "duroscope," invented by Sams and Shaw (U.S. Patent

1 ,789,196-Reissue 18,889), provided magnetizing and pick-up coils in a single

probe, and used a similar wattmeter ar- rangement to measure the iron losses in

the area of cutting tools under the probe.

DeForest (U.S. Patents 1,897,634 and

1,906,551) provided means for measuring

magnetic losses in sheets and tubes under

different testing conditions such that the

indication was influenced first by the

combined electrical and magnetic properties of the material, and second, by a change in, and characteristic of, but one

of the unobserved properties--; for example, the magnetic one. These measurements

were correlated with stresses in the material, in U.S. Patent 1,906,551.

Electromagnetic induction tests in

which plates and tubes were excited by a high-frequency field coil connected to a vacuum tube oscillator were employed by

Kranz (U.S. Patent 1,815,717) and by Mudge

and Bieber (U.S. Patent 1,934,619). The

reaction of the test object (presumably

the magnetic losses) was detected by

changes in the amplitude of the exciting

oscillations in the vacuum tube circuit.

Eddy current losses in the test

object were employed to detune a highfrequency oscillator, one of whose harmonics was heterodyned with a different

harmonic of a standard frequency signal to provide beat signals, in a rod tester described by Dana (U.S. Patent 1,984,465).

Roop (U.S. Patent 2,055,672) placed stan-
dard and test bars in opposite sides of an inductance- type bridge circuit, whose un- balanced output signal was amplified and

applied to a thyratron relay which operated suitable markers to indicate the location of defects on the test object.

Zuschlag (U.S. Patent 2,077,161) re- veals the difficulties of compensating

loss testers for variations (other than

flaws) in test objects and standards, and

for electromagnetic interference from ex- traneous sources, and proposes circuit

improvements to reduce their undesirable

effects. In U.S. Patent 2,098,991, he

proposes an artificial standard circuit

which introduces into the pick-up circuit

signals corresponding to the indications

of a standard test object, with which the

indications of an unknown test object are

! automatically compared during testing.

I Further improvements in the detection circuit are shown in U.S. Patents 2,208,145,

2,329,810, and 2,329,811.

Kinsley (U.S. Patent 2,101,780) designed exciting coil assemblies which proI duced uniform flux densities over the test

area in sheets, bars, and strips, so that

eddy current and hysteresis losses (of

importance in sheet materials for transformers, dynamos, and other electrical

equipment) might be measured in a manner

comparable to standard Epstein tests of

specially cut samples.

Fermier (U.S. Patent 2,389,190) devised equipment whereby tubes and bars

could be subjected to a series of exciting

frequencies, each producing a different

penetration of eddy currents. The voltage

drop across the exciting coil (in the tank

circuit of a vacuum tube oscillator) was

registered, for each frequency, by a point

on the screen image of a cathode- ray oscilloscope, so that the response to several

test frequencies could be observed simultaneously. The indications have been cor- related with properties, such as carbon

content of material in the surface layers.

Thermal Pick-up Methods:

DeForest (U.S. Patent 1,869,336) described several arrangements of coils for

induction heating of tubes and welds,

| whose temperature distribution was i ndi

i cated by isothermal s delineated by melting

of stearin or other suitable temperature

indicators. Somes (U.S. Patent 2,340,150)

proposed the use of high-frequency induction heating with heavy induced currents.

As the induction-heated zone progressed

uniformly along the tube wall, the heavy

currents, upon striking a high- resistance

sand hole or pocket hidden from the sur- face by a layer of homogeneous metal,

caused a burn-out resulting from fusion at the point, thus breaking down the wall of

the tubular object and revealing the

hidden defect.

Mechanical Pick-up Methods:

A novel method of detection was proposed by Burrows (U.S. Patent 1,599,645)

in which a magnetizable test object was placed on a rotatable spindle in a threephase rotating magnetic field. The torque

developed in the object (which acted some- what like the rotor of an induction motor)

was measured by the displacement of the

supporting spindle against a restraining

spring, and was assumed to measure significant physical characteristics of the test

object.

Technical Literature References

on Electromagnetic Induction

Non-Destructive Tests

(1) Konig, "Nondestructive Testing of

Railway Car Wheels and Axles for

Surface Cracks," Oregon Fortschr.

Eisenbahnw. , Vol. 91, December 15,

1936, pp. 504-509.

(2) L. H. Ford and E. E. Webb, "Nondestructive Testing of Welds," The

Engineer, Vol. 165, April 8, 1938,

pp. 400-401.

(3) Anonymous, "Nondestructive Test (Production) for Steel Tubing," Steel,

Vol. 107, October 21, 1940, pp. 38-40, 75.

(4) Ross Gunn, "Eddy-Current Method for

Flaw Detection in Nonmagnetic

Metals," Journal of Applied Me- chanics, Vol. 8, No. 1, March, 1941,

pp. A22-26.

(5) W. Jellinghaus and F. Stablein,

"Nondestructive Testing to Detect

Internal Seams in Sheets," Technische

Mitteilungen Krupp, Ausgabe A. Forschungsber, Vol. 4, April, 1941, pp. 31-36.

(6) A. Trost, "Testing Non-Ferrous Pipes,

Bars and Shapes with Eddy Currents,"

Metal lwirtschaft, Vol. 20, pp. 697-699 (1941); also Chemische

Zentralbl, Vol. 1, p. 801 (1942).

25
(7) W. A. Knopp, Jr. , "Rapid Nondestructive Testing with Cathode Ray Oscilloscope," Instruments, Vol. 16,

January, 1943, pp. 14-15.

(8) F. Forster and H. Breitfeld, "Nondestructive Test by an Electrical

Method," Aluminium, Vol. 25, March,

1943, p. 130.

(9) James G. Clarke and Charles F. Spitzer, "Electronic Locator for

Salvaging Trolley Rails," Electronics, Vol. 17, January, 1944,

p. 129.

(10) F. Forster and H. Breitfield, "Nondestructive Testing of Light Metals

Using A Testing Coil," Light Metals

Bulletin, Vol. 7, April 28, 1944, pp. 442-443.

(11) Anonymous, "Nondestructive Testing,"

Automobile Engineering, Vol. 34, May,

1944, p. 181.

(12) J. Albin, "Salvaging and Process

Control with the Cyclograph," The

Iron Age, Vol. 155, May 17, 1945, pp. 62-64.

(13) John H. Jupe, "Crack Detector for

Production Testing," Electronics,

Vol. 18, No. 10, October, 1945, pp. 114-115.

(14) H. L. Edsall, "Magnetic Analysis In- spection of Metals," Materials & Me- thods, Vol. 22, December, 1945, pp. 1731-1735.

(15) H. Mader, "Magneto-Inductive Testing," Metal Industry, Vol. 68,

January 18, 1946, pp. 46-48.

(16) P. E. Cavanagh, E. R. Mann, and R. T. Cavanagh, "Magnetic Testing of Metals," Electronics, Vol. 19, August,

1946, pp. 114-121.

(17) Vin Zeluff, "Electronic Inspection,"

Scientific American, Vol. 174, No. 2,

February, 1946, pp. 59-61.

(18) J. E. Carside, "Metallic Materials

Inspection," Metal Treatment, Vol.

13, Spring, 1946, pp. 3-18.

(19) G. R. Polgreen and G. M. Tomlin,

"Electrical Nondestructive Testing of

Materials," Electronic Engineering,

London, Vol. 18, No. 218, April,

1946, pp. 100-105.

(20) R. L. Cavanagh, "Nondestructive Testing of Metal Parts," Steel Processing, Vol. 32, No. 7, July, 1946, pp. 436-440.

(21) P. E. Cavanagh, "A Method for Predicting Failure of Metals," ASTM Bulletin, No. 143, December, 1946, p. 30.

(22) R. L. Cavanagh, "Nondestructive Testing of Drill Pipe," Oil Weekly, Vol.

125, March 10, 1947, pp. 42-44.

(23) Howard C. Roberts, "Trends in Electrical Gaging Methods," Instruments,

Vol. 20, April, 1947, pp. 326-330.

(24) C. H. Hastings, "Recording Magnetic

Detector Locates Flaws in Ferrous

Metals," Product Engineering, Vol.

18, April, 1947, pp. 110-112.

(25) Anonymous, "Electronic Comparators,"

Automobile Engineer, Vol. 37, July,

1947, pp. 271-272.

(26) P. E. Cavanagh, "Some Changes in Physical Properties of Steels and Wire

Rope During Fatique Failure," Transactions, Canadian Inst. Mining and

Metallurgy, July, 1947, pp. 401-411.

(27) G. B. Bowman, "Measurement of Thickness of Copper and Nickel Plate,

Monthly Review, Vol. 34, October,

1947, pp. 1149-1151.

(28) R. S. Segsworth, "Uses of the DuMont

Cyclograph for Testing of Metals,"

Electronic Methods of Inspection of

Metals (Am. Soc. Metals), pp. 54-70

(1947).

(29) Charles M. Lichy, "Determination of

Seams in Steel by Magnetic Analysis,"

Electronic Methods of Inspection of

Metals (Am. Soc. Metals), pp. 97-106

(1947).

(30) P. E. Cavanagh, "The Progress of Fail- ure in Metals as Traced by Changes in

Magnetic and Electrical Properties,"

Proceedings, Am. Soc. Testing Mats.,

Vol. 47, p. 639 (1947).

(31) P. E. Cavanagh and R. S. Segsworth,

"Nondestructive Inspection of Mine

Hoist Cable, " Transactions, Am. Soc. Metals, Vol. 38, pp. 517-545, discussion, pp. 545-550 (1947).

(32) Carlton H. Hastings, "A New Type of

Magnetic Flaw Detector," Proceedings,

26
Am. Soc. Testing Mats., Vol. 47,

p. 651 (1947).

(33) Abner Brenner and Eugenia Kellogg,

"Magnetic Measurement of the Thickness of Composite Copper and Nickel

Coatings on Steel," Journal of Re- search, Nat. Bureau of Standards,

Vol. 40, No. 4, April, 1948, pp. 295-299.

(34) A. M. Armour, "Eddy Current and Electrical Method of Crack Detection,"

Journal of Scientific Instruments and

of Physics in Industry, Vol. 25,

June, 1948, pp. 209-210.

(35) Kurt Matthaes, "Magnetinduktive Stahlprufung," (Magneto-Inductive Testing of Steel) Zeitschrift fiir Metal

lkunde, Vol. 39, September, 1948, pp. 257-272.

(36) James Gee, "Testing and Inspection of

Wire Ropes," Mine and Quarry Engineering, Vol. 14, December, 1948, p. 375.

(37) I. R. Robinson, "Magnetic and Inductive Nondestructive Testing of Metals," Metal Treatment and Drop

Forging, Vol. 16, Spring, 1949, pp. 12-24.

(38) Abner Brenner and Eugenia Kellogg,

"An Electric Gage for Measuring the

Inside Diameter of Tubes," Journal of

Research, Nat. Bureau of Standards,

Vol. 42, No. 5, May, 1949, pp. 461-464. (RP 1986).

(39) George A. Nelson, "The Probolog, for

Inspecting Nonmagnetic Tubing," Metal

Progress, Vol. 56, July, 1949, pp. 81-85.

(40) P. Zijlstra, "An Apparatus for Detecting Superficial Cracks in Wires,"

Philips Technical Review, Vol. 11,

July, 1949, pp. 12-15.

(41) G. H. Vosskuhler, "Nondestructive

Testing of the Al-Mg-Zn Alloy Hy-43

by a Magneto- Inductive Method,"

Metal 1, Vol. 3, August-September,

1949, pp. 247-251 and pp. 292-295.

(42) P. Schneider, "Measuring the Wall

Thickness of Light-Metal Cast Parts

With Dr. Forster's 'Sondenkawimeter'," Metal 1, Vol. 3, October,

1949, pp. 321-324.

(43) Theodore Zuschlag, "Magnetic Analysis

Inspection in the Steel Industry,"

Symposium on Magnetic Testing, pp. 113-122, Am. Soc. Testing Mats.,

(1949). (Symposium issued as separate publication STP No. 85.)

27
Symposium on Non-Destructive Testing

table iii.—patents on electromagnetic induction non-destructive tests.

Patent No. Patent Date Inventor

1,599,645 1/26/24 Charles W. Burrows

1,676,632 7/10/28 Charles W. Burrows

1,686,679 10/ 9/28 Charles W. Burrows

1,782,462

1,813,746

11/25/30

6/ 7/31

Helmut Chappuzeau

and Otto Emersle- ben

Carl Kinsley

1,815,717 7/21/31 Hermann E. Kranz

1,869,336 7/26/32 Alfred V. de Forest

1 , OV / , Oj'l LJ 1^/ oo Alfred V. de Forest

1,906,551 5/ 2/33 Alfred V. de Forest

XVC. 10,007

1,943,619

1,958,079

/ / -r/ JJ

1/16/34

5/ 8/34

T A QotTrtc on/1 \/ii"tril J. /I. OdlUS tlUU virgn

F. Shaw

Wm. A. Mudge and

Clarence G. Bieber Arthur E. F. Bilstein

1,984,465 12/18/34 David W. Dana

1,992,100 2/19/35 Wilhelm Stein

2,007,772

2,010,189

2,055,672

I , uOO ,118

7/ 9/35

8/ 6/35

9/29/36

Li./ LLf JO

J. A. Sams and Charles D. Moriarty

Howard T. Hallowell,

T,jr. Harold D. Roop

Archibald H. Davis,

Jr.

2, Uuj, 1 iy

0 720

llj ILJ oO

2 /9 2 /27 of lo/ Of

Archibald H. Davis,

Jr. Fred D. Braddon

1 A77 1A1 z , u/ / , loi A /I 3/3.7 *t/ lo/ oi Theo. Zuschlag

2,084,274 6/15/37 Arthur E. Billstein

2,098,991 2,101,780

11/16/37

12/ 7/37 Theo. Zuschlag

Carl K. Westfield

2,104,646 1/ 4/38 Grover R. Greenslade

1 ill 0 1

A

z, ill, ziu 1/1C/70 o/ Lo/oo Lawrence C. Ebel

2,116,119 5/ 3/38 Alfred Loewenstein

2,124,577

2, 15U,y2z

7/26/38 Horace C. Knerr and

Alfred R. Sharpies

1'\ 1,1 T II,,,, uonaiu Li. nay

2, 162, 710 6/20/39 Ross Gunn

2,186,826

2,208,145 Re. 21,853

1/ 9/40 7/16/40

7/15/41

Robert F. Edgar

Theo. Zuschlag

Ralph W. Atkinson

2,245,568 7/17/41 Robert H. Canfield

Assignee

Burrows Magnetic

Equipment Co. Magnetic Analysis Corp. Magnetic Analysis Corp. Neufeldt und Kuhnke

Betriebsgesellschaft

Magnetic Analysis

Corp. Western Electric Co.

American Chain Co.

General Electric Co.

The Pennsylvania Rail- road Co. General Electric Vapor Lamp Co.

General Electric Co.

Standard Pressed Steel Co.

Steel and Tubes, Inc.

Steel and Tubes, Inc

Sperry Products, Inc. Magnetic Analysis Corp. The Pennsylvania Rail- road Co. Magnetic Analysis Co. U. S. Steel Corp.

Pittsburgh Dry Stencil Co. Anaconda Wire and

Cable Co.

Steel and Tubes, Inc.

General Electric Co. Magnetic Analysis Co. General Cable Co.

Title

Method of Testing Magnetizable Objects

Method of and Apparatus

for Testing Magnetizable

Objects

Apparatus for Testing Magnetizable Objects

Arrangement for Testing

Magnetizable Objects

Method of and Apparatus

for Magnetic Testing

Apparatus for Measuring

Variations in Thickness

of Metallic Bodies Thermal Method of Testing

Metallic Bodies Method of and Apparatus

for Electromagnetic Testing

Magnetic Testing Method

and Means

Apparatus for Testing Metals Method and Apparatus for Testing Materials Method and Apparatus for Testing for Internal Flaws Method of and Apparatus

for Detecting Structural Defects in Materials Testing Flaws and the Like

in Working Materials Magnetic Testing Apparatus Means for Testing Metal

Metal Testing Device Method and Apparatus for Testing Metals for Defects Flaw Detection

Indicating Device for Flaw

Detector Magnetic Analysis Method

Electrical Tester Magnetic Analysis

Electromagnetic Testing of Materials Means for Testing

Apparatus for Determining

Wall Thickness System for Electrically Measuring the Thickness of Metallic Walls, Sheets, and the Like Method and Apparatus for Testing Metal Articles Apparatus and Method for Detecting Defects in Electrically Conductive Objects Apparatus and Method for Detecting Defects in Metallic Objects

Eccentricity Indicator Magnetic Analysis

Method and Apparatus for Measuring Eccentricity

Method of and Apparatus

for Examining Ferromagnetic Articles

8 No

3 No

23 No

30 No

12 No

10 No

2 No

21 No

15 No

8 No

6 No

2 No

7 No

9 No

11 No

5 No

g No

64 No

8 No

18 No

3 No

7 No

13 No

9 No

9 No

23 No

4 No

18 No

36 No

12 No

28
McMaster and Wenk on a Basic Guide

TABLE III—PATENTS ON ELECTROMAGNETIC INDUCTION NON-DESTRUCTIVE TESTS (Continued).

Patent No. Patent Date Inventor Assignee litlc

Number

of

Claims Expired

2,290,330 7/21/42 Emmett M. Irwin Magnetest Corp. Method of and Apparatus No

for Testing Properties of Materials

2,311,715 2/23/43 Harold C. Ghorne Apparatus for and Method 26 No

of Detecting Haws in Rails and Other Objects

2,315,943 4/ 6/43 Loren J. DeLanty Sperry Products, Inc. Means for lesting lubes 2 No

2,329,810 9/21/43 Theo. Zuschlag Magnetic Analysis Electromagnetic Inspection 19 No

Corp.

2,329,811 9/21/43 Theo. Zuschlag Magnetic Analysis Electromagnetic Inspection 19 No

Corp.

2,334,393 11/16/43 Lyle Dillon Union Oil Co. Determination of Magnetic 10 No

and Electrical Anisotropy

of formation Core Samples 2,340,150 1/25/44 Howard E. Somes Budd Induction Heat- fault- lesting Articles of 3 No

ing, Inc. Electrically Conductive

Material

2,353,211 7/11/44J Theo. Zuschlag Magnetic Analysis Electrical Analysis 3 JNo Corp.

2,389,190 11/20/45 George F. Fermier Reed Roller Bit Co. Testing Means 4 No

2,398,488 4/16/46 Theo. Zuschlag Magnetic Analysis Magnetic Analysis 4 No

Corp.

2,489,092 11/22/49 C. V. Larrick General Electric Co. High-Frequency Surface Test- 6 No

ing Instrument

2,489,920 11/29/49 F. C. Michel General Electric Co. Metal Detector 5 No

2,490,554 10/15/46 Harcourt C. Drake Sperry Products, Inc. Flaw Detector for Tubing 4 No

2,491,418 12/13/49 C. H. Schlesman Socony-VacuumOilCo., Automatic Inspection De- 2 No

Inc. vice 2,495,627 1/24/50 D. E. Bovey General Electric Co. Method for Sorting Metallic 1 No

Articles 2,502,626 4/ 4/50 Morris L. Mages Magnaflux Corp. Electronic Metal Detector 4 No

29
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32
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977 Issued January 1981. '" UCu

EDDY CURRENT TESTING: PRESENT AND FUTURE APPLICATIONS

IN THE FERROUS METALS INDUSTRY

Richard Moyer

Carpenter Technology Corporation

Reading, PA 19603

The scope of these remarks is to present the past, present, and future

applications of eddy current testing in

the ferrous metals industry, or more

specifically, in the basic steel producing

industry. The source of the data for the

review of the past is a survey conducted

about ten years ago by the American Iron

and Steel Institute. The current and

future information originates from discussions and correspondence with members

of the AISI Technical Committee on Nondestructive Testing and Inspection Systems.

I am grateful for the opportunity to review for this particular audience where

we have been, where we are, and where we would like to go. It is apparent that the

participants of this workshop and the

National Bureau of Standards will have a strong impact on these future directions.

It is forums like these that will pilot

advancements into useful and practical

channels to the benefit of us all, the

steel industry included.

Almost twelve years ago, the newly

formed Institute committee surveyed the

steel industry NDT practices through a series of questionnaires. The companies

reporting provided information on a total

of 313 NDT inspection systems. These

involved four product types: bar, plate,

semi-finished, and tubular products. The

distribution of these among the major NDT

disciplines is shown in the first table.

An assessment of equipment reliability was reported in the survey. Re- spondents were asked to judge the reliability of a system as excellent, good,

fair, or poor. To these, numerical values

4 through 1, respectively, were assigned.

The resulting average ratings for the

various types and methods are shown in the

second table.

The prominent feature of these data

is that eddy current testing had appeared

to reach a maturity as long as ten years

ago from both an application and a re- liability standpoint. It certainly was

one of the big five of NDT.

Table 1. Number of Systems

Product

Form

Eddy

Current

Liquid

Penetrant

Magnetic

Particle

Radiological

Ultrasonic

Bar 31 16 17 Plate 5 18

Semi -Finished 17 38

Tubular 48 17 26 22 58

Table 2. System Reliability

Product Eddy Liquid Magnetic Radio- UltraForm Current Penetrant Particle logical sonic

Bar 3.2 3.0 3.4

Plate 3.4

Semi-Finished 3.1

Tubular 3.2 2.4 3.1 3.1 3.2

33
But what about today? Has eddy

current testing extended any horizons?

Has it fulfilled any new application

needs? Is it more sensitive? More

accurate? More reliable? Do the developments of the last ten years signify a truly expanding technology?

It is unfortunate that a similar

statistical survey reflecting today's

usages is not available to give quantitative answers to these questions. A survey is not needed to fill in one of the

blank spaces of Table 1. Eddy current

inspection of semi -finished billets, both

round and square, has become an accepted

technique, routine in some mills.

The topic of semi-finished product

testing leads directly to an expanding

related area of application; in process

testing. This is simultaneous ECT while

another phase of the manufacturing cycle

is being performed. Utilization of the

technique has been made in conjunction

with bar straighteners , wire drawing

blocks, and more recently, hot rolling

mills for both tubular and solid products.

In addition to inspecting its own products, the steel industry has found ECT

useful in defect detection in items of its

processing equipment. Rolling mill rolls

and crane hooks are notable examples.

The steel industry has found eddy

current testing techniques applicable for

uses other than flaw detection. The more prominent of these are coating thickness

measurement and sorting. The thickness

and uniformity of copper plating on cold

heading coil stock is reliably determined

by this technique. Sorting for separation

of grade, hardness, size, or other feature

by eddy currents is a field equally as large and as important as flaw detection.

The problems associated with today's

ECT applications in the steel industry can

be grouped into two categories: operational and performance. "Operational" is

concerned with the ease or convenience of

employing the method, while "performance"

concerns the sensitivity, accuracy,

reliability, and/or effectiveness of the

test.

One of the principal operational

difficulties is the loss of productive

time during set-up and calibration for

stock size change. This problem exists

for both encircling coil and rotating

probe machines—obviously the more complex

the apparatus, the longer the downtime.

This is not as serious a disadvantage for

in-process testers as for separate test

stations, for presumably, adjustments to i the tester could be made while the pro- I cessor is changed.

I Convenience of calibration is another

operational problem. This may be more I difficult on an in-process tester than on i a test station, especially if a reference j calibration piece with real or artificial \ defects is involved.

Another operational difficulty is

maintaining the proper transducer (coil) to product spacing and alignment despite the

influence of temperature extremes, me- chanical handling system irregularities, I crooked ends, etc. The latter could be so I severe as to cause damage or excessive < wear, thus shortening the preventative i maintenance cycle. Ease of maintenance

is, of course, another operational con- sideration.

In considering performance problems,

relability must be listed prominently. Is it a certainty that, when set to detect

.008 inch seams, an eddy current tester

will not accept a bar with one .012 inch [ deep? Or, will a bar with a harmless

scratch of .004 inch be rejected? Also,

in sorting mixed steel, is the separation

absolutely correct? The steel industry is not sure.

I Another performance problem is

J accuracy of calibration. There are many

j types of artificial defects used for

calibration or set-up purposes. The true

correspondence of the eddy current re- I sponses to them and to natural defects is |j either unknown or something vastly' dif- ! ferent from one to one. This lack of

correlation is also influenced by the way

j the artificial defect was made--electric

discharge machining, mechanical metal t removal, manual filing, etc. The shape of

the calibration defect is not always rep- !, resentative of the true defect it mea- sures, as with a hole drilled through the

wal 1 of a tube.

Another performance deficiency is the

inability of encircling differential coil

systems to reliably detect and accurately

evaluate continuous defects. This need is severely felt on installations where

rotating probes are unsuitable, such as on i a hot rol 1 ing mi 1 1

.

34
Test sensitivity and its relation to

inherent noise creates a performance

problem. Often a realistic accept/reject

level cannot be achieved with eddy current

test equipment because that level is

submerged in background noise. Unfortunately, phase adjustments do not always

provide a differentiation to solve the

problem.

A performance feature not yet adequately addressed by equipment designers

is the lack of clear relation between the

instrument's readout and the true nature,

size, orientation, and location of the

anomaly disclosed.

A performance consideration needing

attention concerns the inspection of shapes

other than rounds. There is a need to inspect the corners of square billets as critically as the faces. Conversely,

there is an equal need to inspect the

faces of hexagonal bars to the same degree

as the corners.

Obviously each of the problem areas described earlier suggests a future need.

Accordingly, only those having a pressing

urgency will be repeated in this section

devoted to future directions. The greatest need is a reliable eddy current

instrument that will reliably detect and

accurately evaluate continuous, as well as intermittent defects, in hot rolling mill

product which is at 2000 °F as it leaves

the mil 1

.

Farther, toward the horizon, I visualize the utilization of the miracles

of microelectronics in signal processing

and pattern recognition to lead us out of

the realm of unreliability and lack of

sensitivity. Other solutions to these

problems might be found in pulse techniques

or more sophisticated phase modulation

and/or frequency analysis. Perhaps

computer techniques will provide a hard

copy printout of an eddy current test of a billet showing exactly where each flaw is

located and how severe it is. Or, perhaps

instead of the print-out, the computer

will provide guidance to the grinder so that it may remove each flaw.

In summary, to answer the questions

posed earlier, the science of eddy current

testing has made progress in the last ten years, but there is a much longer road

ahead.

Discussion

Question (Mr. Weismantel, General Electric

Co.): I have two questions, both of them

involve opinion more than anything else.

Number one, as you know, we looked at

quite a few different mills with respect

to the NOT capability. One of the things we noticed, especially in the eddy current

area more than the ultrasonic area, was

the lack of standardization between mills

within the same industry as far as how the

process was applied and how it was con- trolled. That is one of the reasons that

we, as a purchaser, come out with specifications which you might think are overly demanding or are different than someone else would expect. Who do you

feel should try to get standardization

within the steel industry for a product of

this sort?

Answer (Mr. Moyer): I would answer that

by saying that the customer pushes the

producer into standardization. I say that

because the status of standardization in

ultrasonics, for example, is much further

advanced; in other words, the steel

industry performs inspections to cus- tomers' specifications at least 100 times more often in ultrasonics than we do in

eddy current. That is because of customer

insistence. I must confess that this push

for standardization has been at the

customer's impetus. Although eddy current

testing in our mills predates ultrasonics,

personnel qualification, standardization,

and purchasing specifications to quantitative levels in eddy currents are lagging behind ultrasonics. Perhaps, we take the course of least resistance; if our customer says, you have got to do it

this way or we will not buy from you, we

tend to do it. In-house, we prefer to use eddy currents because of its economic

aspects. We need not be quite as rigorous

with standardization if we are satisfying

ourselves, compared to what we would be if we were satisfying the customer.

Comment (Mr. Weismantel): It would appear

that there is no attempt to establish a stable process between different mills.

If we had found more standardization

between the different producers, we would

have more guidance as to how we establish

our specifications.

Comment (Mr. Moyer): It goes a little

deeper than that. There is a question in

35
my mind, perhaps it does not exist in Dr. McMaster's, but I am not sure what eddy

currents are really sensitive to. I am

not even sure whether they respond to the

absence of metal or whether they respond

to the work hardening around an artificial

notch. Ultrasonics is a little different.

Maybe I am gilding the lily a bit, but at

least ultrasonics response has a stronger

sensitivity to the reflecting area of a calibration notch providing it is properly

oriented. We have a greater confidence in

quantitative ultrasonic results than we do

in eddy current results.

Comment (Mr. Booth, Bethlehem Steel Corp.):

I would like to respond to that. I think

customers bring part of the trouble on themselves in that they buy from several

mills in irregular sequence at the lowest

price. Whatever is coming down the

production line, you may have steel from

different mills and several companies

going to production at such a rate there

is no way of correlating results of tests.

And, of course, AISI does not put any

specifications on the magnetic properties

of materials at all. Consequently, unlike

Forster, who initially encouraged his

customers to buy an entire melt or enough

steel for a year or two's products and

calibrate the hell out of it, with this

random material coming down the line there

is no possibility of realistic testing.

So a part of the problem is the customer's

fault; he does not demand a whole lot.

Comment (Mr. Weismantel): I have a second

comment. You brought up the problem of

how eddy current response relates to an EDM notch versus a response from a flaw of any

particular nature. I look at the calibration of an instrument as a control to

attain uniform inspection sensitivity. It

is our responsibility as a purchaser to try to determine what that response level or rejection level should be, relative to the types of flaws we think are most

damaging to us.

Comment (Mr. Moyer): I am delighted you

are assuming that responsibility.

Comment (Mr. Weismantel): The point I am trying to make is we do like to see standardization in the calibration of the

equipment. If there is one area the steel

industry has a lot of standardization in,

it is in their magnetic analysis equipment.

That is one thing I have found true across

the field; but beyond that, standardization stops.

Question (Mr. Berger): You indicated one

of the big new uses for eddy current

testing was in sorting materials, yet one

of the problem areas you mentioned was the

difficulty in sorting materials. Could

you expand upon that? Are the things you j measure too close in electrical properties

for you to make an adequate separation?

Answer (Mr. Moyer): Sometimes, that is

unfortunately true.

Question (Mr. Berger): Could you give us an idea of what level of repeatability or accuracy or sensitivity you are looking to achieve the sorting specifications you

need?

Answer (Mr. Moyer): Well, my particular

company makes a variety of steels, it

numbers in the hundreds of grades, and some of those defeat any comparator when

you try to separate them; for example,

type 316 stainless and 316 low carbon

stainless. There are some disastrous

mixes for the automotive industry, for

example, and I feel it is essential that

the mix be separated with absolute cer- tainty.

Comment (Mr. Hentschel): There is no reason to have disastrous mixes any more. We manufacture a microprocessor control

that will sort through frequencies and so forth on the signature of those steels.

The question comes down to the grosser

differences and not the disastrous ones.

Comment (Mr. Moyer): This is why I am delighted this forum is assembled. These

things are being brought to light.

Comment (Mr. Bugden): I think the point

Bob McMaster made is pertinent to this, as far as all the variations, not only in

chemistry but in processing of various

steels. Certainly since we are talking

about calibration, I think we can see how

difficult it is. I would say that it is

possible to sort mixes, but it is difficult

in stainless steel or alloys to calibrate

samples, and carry them over.

Comment (Mr. Hentschel): I want to add to | the point that Dr. McMaster made. In Europe, they did respond in the manu- facturing processes; they would be willing

to change the process to allow an optimum

set-up and rearrange the manufacturing to facilitate testing. When you try to suggest it here, they think you are crazy.

36
Comment (Mr. Moyer): Sometimes the

suggestions that are made are very ex- pensive. For example, a customer suggested

that testing occur at a given intermediate

size; that means we would have to interrupt

a hot rolling cycle to get it at that

size, provide a surface sufficient to accept the test, test it, and then re- introduce it to the hot mill.

Comment (Mr. Hentschel): In the automotive industry, there are examples where

specs for the part manufactured do take

testing into account. It is beginning to get better.

Question (Dr. Taylor): The Japanese are supposedly rather advanced in automatic

steel production, at least that is what we

read in the paper. Have they generally

used eddy current testing in their steel

mills?

Answer (Mr. Moyer): I cannot answer, I am

sorry. I am not familiar with their

techniques.

Question (Mr. Weismantel): With regard to the accuracy of calibration, you mention

the lack of correlation between artificial

defects and real defects. This is quite

understandable. But how well does one do

when comparing the response from two or more artificial defects, presumably made

identically. Are these reproducible?

Answer (Mr. Moyer): We have not found

them to be as reproducible as we would

like. Unfortunately, the most recent

reproducible artificial defect that we

have found has been a hole completely

through the wall of a tube, which pyschologically is very unacceptable to a customer. But a lot of it has to do with

what we said earlier, what are eddy

currents sensitive to? It depends on how

you manufacture the defect and how you

standardize those processes, really, to make artificial defects reproducible.
rsPeitea
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg , MD, November 3-4, 1977. Issued

January 1981

.

EDDY CURRENT STANDARDS IN NONFERROUS METALS

Carlton E. Burley

Reynolds Metals Company

Richmond, VA 23261

This presentation is not intended to represent the aluminum industry or any

specific company, but to give some personal thoughts based on 20 years of

experience in NDT activities.

When I first started my involvement

in NDT, being trained in physics and electronics, one thing that puzzled me was the

emphasis on ultrasonics and radiography

and the exclusion of eddy currents for

defect identification. Soon I learned

that it is difficult to convince metallurgists and quality controllers that an eddy current "trace" has meaning.

Let us examine how we use electrical

conductivity and how standards are related

to this parameter. We use quantitative,

as well as qualitative, measurements of

conductivity.

Figure 1 shows the range of

electrical conductivity for some cast

aluminum alloys. In all cases, the

conductivity extends over a considerable

range. The as-cast condition is shown as AC, annealed material as 0- temper and, in some cases, intermediate tempers are

given. Notice that there is usually an overlap among alloys.

In many cases, eddy current defect

inspection has been oversold. It is not difficult to find surface cracks and sur- face scratches, but too often such imperfections mask the more serious problems of

internal discontinuities and lead to ex- cessive rejection rates. To avoid this,

the sensitivity is reduced and everything

passes inspection.

Eddy current inspection for material

defects, in my opinion, requires techniques and operator competences that are usually not available in the typical

plant.

Most of the processes that we use today have developed from laboratory and

research investigations. This is, again,

one of the characteristics of eddy current

technology: the people who best understand electromagnetics and the behavior of

materials are in the research laboratories. With such expertise, we are able to

do much more with eddy currents in the

laboratory than can be practically

translated to our plants.

Most of the applications of eddy cur- rents that are used are those involving

the measurement of electrical conductivity. These tests are usually performed manually.

COHDUCTIVIIY (X IACS

Figure 1. Conductivity of several aluminum casting alloys.

A similar situation is shown in

figure 2. These are conductivity ranges

for several wrought aluminum alloys.

This is one factor to be considered

in standards for sorting; we can use eddy

currents for sorting only if we know

something more than the conductivity.

A further limitation is necessary for

the heat treatable alloys. Figure 3 gives

conductivity versus strength values which

are typical of a 7XXX or 2XXX alloy.

Notice that we have a cycle. We can start

with the quenched material, proceed to naturally aged, then to artif ical ly aged

39
and overaged and come back to an annealed

condition.

5056

-H3S <

20 it

-T i

-B18 (

CONDUCTIVITY (I IACS)

Figure 2. Conductivity of several wrought

aluminum al loys.

•*SKT

NATURALLY AGED

CONDUCTIVITY

Figure 3. Conductivity versus strength

cycle of heat treatable aluminum

al loys.

There are many double points here. For example, if the conductivity is about

midway or somewhat less than the peak at the artificial aging position, on which

side of the peak are you? Are you heading

for the overaged condition or are you on the natural aging side? This is important

if the conductivity is to be a criterion

for corrosion properties.

Thus, strength or some other mechanical properties are also required in

order to apply eddy currents to identify

temper for heat treatable materials.

These measurements, however, are a good example of an area in which conductivity standards have application. To measure electrical conductivity, you need a

resistance or conductivity standard. The

standards generally used are aluminum bars

or rods measured with a Kelvin bridge.

Originally, when we set up our electrical

standards lab, we furnished NBS with several samples for measuremnt and have subsequently used them as standards. These

standards have been checked periodically

by NBS or another qualified laboratory.

Such calibration has been a direct current

measurement. Thus, when we meet a cus- tomer's specification for percent IACS, we

are certifying it against a DC standard

since we calibrate our eddy current instruments to such DC standards 1

.

Our lab and plant standards are cut

from Kelvin bridge measured specimens;

these are used to certify working standards which are placed on each test

instrument.

In many cases, we are not concerned

with the absolute conductivity. In sorting, for example, only a conductivity difference may be needed. But if you need to measure a sample precisely, a question is: how accurately can you measure on an eddy

current instrument?

It has been our experience that an eddy current technique is accurate to 1-2

percent of the reading. If a customer

requires more precise conductivity certification (such as ofter required for an electrical conductor alloy), we would use

a Kelvin bridge or equivalent DC

measurement.

Errors can also creep into a calibrated eddy current conductivity meter. For example, assume you are using standards of 35 percent IACS and 50 percent

IACS for a two-point calibration. If then

you want to measure material having conductivity of about 25 percent, it is easy to have an unknown error since you have not verified the instrument linearity below 35

percent. In conductivity measurements,

one calibration standard should be below

the lowest value you wish to read and the

second should have higher conductivity

than any specimen you wish to read. For best accuracy, we recommend using two

standards which are relatively close to- gether; e.g., 30 and 40 percent should be

used if you are measuring material in the

range of 33-36 percent conductivity. The

closer the standards bracket the sample,

the greater is the accuracy obtainable.

l ASTM B193

40
NBS should continue to provide primary reference measurements for DC conductivity and resistivity. As an expansion,

a facility to provide comparison measurements at 60 kHz and 100 kHz (common frequencies used for eddy current con- ductivity meters) would be a valuable aid

to the NDT community.

Another measurement that is related

to conductivity is the cladding thickness

i measurement, for example, of alclad alI loys. Normally, specimens are cut from the corners of plate and sheet and measured

optically to provide verification of cladding thickness. This is a slow process for

large amounts of material and also does

not provide a way to monitor or measure

cladding thickness over an entire plate.

Eddy current phase relationships can

| be used to measure cladding thickness on aluminum alloys. Special probes had to be

developed2. We found that with frequencies of 50 to 500 kHz we can ac- curately determine cladding thickness by

an eddy current conductivity measurement.

tive test; a cladding variation of 0.0001

inch can readily be measured.

As an example, figure 5 shows that by

using a higher frequency, full-scale calibration has been reduced to three mils.

The calibration is very close to being

truly linear.

3.5

x 1.5 a

<

0.5

STANDARD CALIBRATION ALCLAD 7075-T6 Machined Sample

Phasemaster B Frequency: 500 KHz

1 1 1 I \

Meter Reading Figure 5. Calibration curve for alclad

7075-T6.

Figure 4 shows a calibration curve

for alclad 2024-T3. We are using a 200

kHz frequency; notice that we can spread

out a five to ten mil cladding thickness

over the entire range of the meter. This

is a zero to 50 division meter.

11.0

5.0

STANDARD CALIBRATION

ALCLAD 2024-T3 Machined Sample

Phasemaster B Frequency: 200 KHz

\#

i i i i \ 0 10 20 30 10 50 Meter Reading

Figure 4. Calibration curve for alclad

2024-T3.

This range could be further extended.

One feature about an instrument of this

type is that you can, by zero suppression

and range changing, develop a very sensiWhere do you obtain standards for

this type of measurement? Figure 6 shows

alclad aluminum which has been machined to provide a calibrated step block.

Figure 6. Machined step block for clad

ding thickness standard.

2 Dr. C. Dodd of ORNL provided valuable guidance and assistance.

41
First, it is necessary to have a representative section. Initially, the material was scanned to be sure that the cladding was uniform. Then metal lographic

sectioning was used to check the innermetallic layer for the presence of diffusion or any other metallurgical effects

that could invalidate the conductivity

measurement.

Once assured of a representative

sample, a series of steps of roughly one

mil each were machined from the cladding

side. Then, a narrow slice of material was cut out of the center of the machined

sample and the cladding thickness measured

metallographical ly at each step. The edge

pieces provided two standard step blocks

from each machined specimen.

The above is an example of developing

an in-house standard for a specific application. The development of universal

standards for all alclad products would be

a major undertaking and one that we have

not recommended. A procedure for developing such standards would probably be the

most useful activity for NBS.

The above techniques have also been

used to sort clad and unclad material such

as sheet, plate, and tubing.

Lift-off techniques are used in the

evaluation of products coated with non- conductive films. For these measurements,

standards have been produced in-house (or

by the equipment supplier) using optical

thickness or film weight measurements. In the range of very thin base materials,

such as coated aluminum foil, the lift-off

method gives erratic readings. The mea- surement problems are compounded by the

need to use very high frequencies to avoid

measuring thickness variations of the

foil.

Thickness measurements of aluminum

foil can be readily accomplished with eddy

currents. For example, 0.0005 inch foil

is laminated to 0.0035 inch paper stock.

Variations in the thickness of the foil can be a cause for rejection of this product. If there is a uniform or gradual

change in thickness of the foil, the product is acceptable. But if there are periodic thickness variations of the order of

15 micro-inches or greater, the material

can be rejected.

We have found that a 500 kHz eddy current test using a probe with a small diameter flat coil can readily measure these

thickness changes. Some of the foil/paper

laminates were brought to the NBS Dimensional Technology Section where the thickness variations were confirmed with a laser interferometer. We are confident

that our system can measure foil thickness

variations to one microinch. The several

j samples that were measured at NBS are used

in our laboratory and our plants for

internal standards.

• - The above discussion has primarily

been based on measurements related to con- ductivity changes in aluminum and aluminum

alloys. The eddy current instrumentation

is either amplitude or phase sensitive i using a single or double probe configuration. Standards are, basically, speci- j mens having known conductivities and/or

known thicknesses.

The next area to be discussed is use of eddy current techniques to locate and

define surface and internal discontinuities. Standards required and used in

this area raise somewhat different problems than previously discussed.

One area for using eddy currents is

location of edge laminations in plate. As aluminum is rolled to plate gauges from

the original ingot thickness, edge laminations or roll-over can develop; even though edge trimming occurs, an edge crack

may be present in the final product.

While ultrasonic techniques are frequently

used, these require the plates to be re- moved from stacks and individually measured. Using a small flat eddy current

probe, cut edges of stacked plates can be i readily scanned for cracks. Standards are required to set test sensitivity levels;

plates with known laminations are used.

Another application that should be more widely used is inspection of tubing

and pipe. While an ASTM procedure for j this method has been published, we find

that the use of notches and drilled holes

is often inadequate for the specific ex- aminations required. More frequently,

standards used are materials with typical

production defects—inadequate welds, ID and 0D voids , etc.

At present, there is not a large

amount of eddy current defect inspection

done in the primary aluminum industry.

One reason it is not used is the unfortunate oversell of equipment which, upon

full evaluation, proves to be not designed

or able to meet requirements. If it works

with steel or copper, it will not neces- sarily work with aluminum.

42
An area in which I would like to see increased effort is measurement of materials at high temperatures. Continuous

casting processes for both sheet and rod are becoming common; we are going to need

better techniques for monitoring such

products. Eddy current techniques have

so much potential, at least theoretically,

that improved detection procedures and

data processing methods should have a good

change of commercial success.

Part of the problem is that we try to use conventional techniques since we know

we are limited in the amount of money we

can spend on fundamental research. Fre- quently, the choice is to work with instrument manufacturers, which requires

full cooperation and interchange between

the producer and vendor. This is not

always possible because of proprietary

requirements. An active program at NBS

should help the development of eddy cur- rent inspection devices.

In conclusion, seminars dedicated to free exchange of information among users, potential users, and vendors, such as displayed at this workshop, should be a good

stimulus to better understanding and

utilization of eddy current techniques.

Discussion

Question : You mentioned you had Kelvin

bridge samples checked by NBS and that,

subsequently, these were rechecked. Over

a period of time, did the conductivity

change?

Answer (Mr. Burley): In some cases, yes.

One of the problems is that to cover the

complete range of conductivities, stable

alloys are not always available. However,

most of the standards have remained con- stant for many years. Wear and scratches

are usually the prime cause for

replacement.

Question : The Kelvin bridge is a DC determination of an AC quantity. Is there a significant variation between bridge

samples?

Answer (Mr. Burley): For the purposes of

certification, most requirements refer to ASTM B193, which is a DC technique. Our

philosophy has been to calibrate our eddy

current meters against DC resistance

standards. Since eddy currents measure

only near surface conductivity, while DC measurements are volumetric, there could

be considerable difference if surface

structure and chemistry are significantly

different from volumetric properties.

Samples which display such properties are

not used for our standards.

Question : What order of magnitude of

variation are you seeing in your

standards, say, for the worst case?

Answer (Mr. Burley): One or two percent.

Question : If you take a general piece of

plate or sheet and measure conductivity

variation over the sheet that is supposed

to be homogeneous, how large a variation

do you get?

Answer (Mr. Jones): Several percent. We

would not be surprised with two percent.

When you approach the butt end or head end

of the original ingot, you are quite

likely to find a larger variation.

Question : What sort of variation do you

get in the middle away from the ends?

Answer (Mr. Burley): Not more than 1-2

percent when you get to the final rolled

product, say quarter-inch thick plate. If

larger variations are found, they will be

due to changes in chemistry, differences

in cold working, or differences in thermal

treatment. My earlier figures showed

these variations may be several percent

IACS.

Question : On production line, how do you

control temperature so that it does not

produce errors far greater?

Answer (Mr. Burley): For reporting or certification purposes, we measure samples

in the laboratory and allow them to come

to the same temperature as the standards.

When measurements are made in the plant,

samples may not be at the same temperature

as standards; only qualitative values can

be obtained. But in most cases, you are sorting and you are not too concerned since

all readings are being shifted in the same direction.

43

National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977. Issued

January 1981.

EDDY CURRENT INSPECTION OF GAS TURBINE ENGINES

Robert A. Betz

NDT Development

Pratt & Whitney Aircraft Group

United Technologies Corporation

East Hartford, CT 06108

All of the many capabilities of the

eddy current inspection method are used

in the development, manufacture, and

maintenance of gas turbine engines. It

is used for flaw detection, material and

coating thickness measurements, material

sorting and identification, metallurgical condition monitoring, and electrical conductivity measurements. It is

used to inspect raw materials, parts

during manufacture, and as a service

routine, some are unusual; many are common to all users of the method and

some are peculiar to the industry.

Anything approaching a complete discussion of its applications would fill a good sized book. For my purposes here,

a few examples of the kinds of application that it finds in the field of gas

turbines may serve to illustrate its

usage.

The performance of a gas turbine

engine improves as the temperature of

the exhaust gases increases. The

maximum operating temperature, however,

is limited by the turbine parts, particularly the first turbine blade. There

are, of course, limits to the temperature increases that are possible

through the development of improved

materials. An alternative approach, air

cooling, has therefore been extensively

developed over the past ten to fifteen

years. Here the blades are made with

internal passages through which relatively cool air is circulated. Such

schemes require that the wall thickness

be measured because the blade wall

should be as thin as possible for most

efficient cooling, but, since the blade

is highly stressed, too thin a wall can lead to failure.

The cross-section of an air-cooled

blade is shown in figure 1. In this

particular part, the thinnest, and

therefore the most critical, area is at the trailing edge cavity as indicated.

The shape of the part is that of an airfoil so that it has a nonconstant

Figure 1. Cross section of a portion of an air-cooled turbine blade.

geometry on each side and from end to

end. The minimum wall can occur on either side and at any location along

the length of the blade, thus making an ultrasonic measurement impractical. The

problem of testing blades was solved

using a phase-sensitive, eddy-current

instrument of the type developed by Dodd

at the Oak Ridge National Laboratory.

The use of a purely phase-sensitive

instrument virtually eliminates the

lift-off problems that arise because of

the geometry and makes it possible both

to locate the minimum wall and to measure its thickness. The technique

has proven to be rapid, reliable, and

capable of measuring the thickness

within ± 0.002 inches.

Another application is the measurement of a multilayer coating. An experimental coating system using three layers

is shown in figure 2. The problem was

to determine if each layer fell within

the prescribed thickness range. The

first layer, applied directly to a nonmagnetic nickel-based alloy, is very
high in nickel and therefore magnetic.

The middle layer, being a mixture of the

nonconductive top layer and the magnetic

first layer, is less strongly magnetic.

The properties of the various layers

suggested that an eddy current test

might be used to make the measurements.

Because the coating system is a complex

one, an impedance analysis type of

instrument with a storage oscilloscope

readout was used to study the problem.

Figure 3 shows the impedance

THIRD LAYER- CERAMIC

(NONCONOUCTIVE)

MIXTURE OF 1ST I 3RD LATER MATERIAL'. (MAGNETIC)

FIRST LATER

HIGH NICKEL MATERIAL

(MAGNETIC)

Figure 2. Three layer coating.

FIRST LAYEU WAX THCC/JESS

FlfST LAYER IMA*.

nevr l THIRD

LflYEftS lY»ft-»t SECOwD L Ay Eft miu.

FlSiX, SECOND! THICO LAYERS

BASE (MATERIAL

-FIBS I f SECOMD LAYEti MliO. THIRD LAYER MO*. BOUVDED AREAS A6E ACCEPTABLE ZONEC FOR THE. COATIUC COmEluATIOWS INDICATED.

Figure 3. Impedance plane response for

multilayer coating.

plane relationships of various thickness

combinations of the three layers. The areas defined by these maximum and

minimum points represent an acceptable

thickness for each layer. Since the areas do not overlap, the acceptability

of the thickness of any layer can be

determined, provided that the lower

layers are within their required thickness range.

Gas turbine engines use high quality bearings which are required to have

a long life; any possible premature

failure is cause for concern. During

grinding of the bearing races, localized

overheating of the surface can sometimes

occur. This overheating results in

material anomalies. Depending on the

conditions that generated the anomaly, a number of undesirable metallurgical

changes can occur. In the worst case, the area includes both retempered and

rehardened material while in the simplest

case only a residual stress field

results. Because these bearings operate

at very high stress levels, any of these

conditions can result in a premature

failure. Fortunately, all of changes in

metallurgical structure result in a local change in the permeability of the

material so they can all be detected

with an eddy current test. Further,

each condition has its own characteristic

response by which the eddy current test

can identify the condition that is

present.

Up to this point, we have been

discussing manufacturing inspections.

However, eddy current methods are also

widely used for service inspections. In fact, the majority of the flaw detection

applications are in this area.

Gas turbine engines, as with any

rotating machine, are subject to vibration and the resulting fatigue

damage. For most parts, fluorescent

penetrant inspection is used to detect

this damage, but there are some cases where this method is not satisfactory.

One of these is in the root of fan

blades where vibration gives rise to both fatigue damage and a mild surface

galling. The latter condition interferes

with penetrant inspection because it

tends to close the surface opening of

the damage. An eddy current test is

therefore used on these parts. Specially

contoured probes are used to maintain

coil position and alignment because the area of interest is adjacent to a fillet

radius as shown in figure 4.

A major advantage of the eddy current

method in service inspection is its adaptability to remote inspections. By using

some ingenuity, it is often possible to make special probes which can be used to perform internal inspections without

engine disassembly. This kind of application does not always allow maximum

sensitivity to be obtained, but where

adequate sensitivity can be obtained, it saves the considerable cost of teardown

and rebuilding.

As the examples cited may have

indicated, there are a wide variety of

46
FATIGUE CRACK

LOCATION

Figure 4. Fatigue crack inspection of fan

blade roots.

eddy current applications in the industry,

and to cover this range, a considerable

diversity of equipment is required.

While it would be nice to have one piece

of equipment that would be all things to

all tests, this does not appear to be

very likely. For the most part, there

does seem to be commercially available

equipment that can solve those problems

that lend themselves to the eddy current

approach; it is just that the greater

the number of jobs to be done, the

larger the number of different instruments one must have available. The fact

that so many jobs can be done should not

be taken as meaning that new equipment

developments are not needed. With new

or improved equipment and techniques, it

may be possible to find more applications or to significantly improve

those that are already in use. Improved

sensitivity to subsurface flaws, for

example, would be a welcome improvement

in certain applications. For improved

capabilities, however, most users must

depend on the equipment manufacturers;

few users are in a position to develop

new instrumentation. Even when a user

does have the capability to make new equipment developments, the chances of

such equipment being used outside his own facility are slim unless commercial

exploitation follows.

The area of reference standards is an extremely large one. The desirability

of having traceable standards is quite

obvious, but the problems associated

with such a task appear formidable. The

way appears relatively easy in only a few cases. Let us consider the reference

standards required for a few typical

appl i cations.

The checking of aluminum alloys for

hardness requires good conductivity

standards. This is an area that is cur- rently being studied and that is to be

presented in more detail by others.

There seems to be little reason why such

a program should not eventually be

successful

.

How one establishes uniform reference

standards for other types of tests is

not so apparent. Consider coating

thickness measurements as an example.

If only relatively few (say a couple of

dozen) combinations of base material and

coating need to be considered, the

problem would probably be manageable.

Unfortunately, this is not the case. In

a large company, there may be as many as 30 or 40 base material -coating com- binations that could require measurement,

exclusive of nonconducti ve coatings.

Throughout the country, the number of

combinations must be gigantic. Then,

there are always the unusual cases to be

considered, such as the three layer

coating discussed earlier.

Even more difficult from the standards

point of view are tests such as the

blade wall thickness measurement that

has been discussed. Here, the only

practical calibration is an indirect one because one cannot make a direct mechanical

measurement of the reference masters.

To do so requires removal of the opposite

wall which changes the eddy current

response. To calibrate this inspection,

three blades were chosen that gave high,

low, and midrange response. Using these

to establish a uniform instrument setup, enough parts were sectioned and

mechanically measured to establish a calibration curve that related actual

wall thickness and eddy current instrument

meter reading.

Reference standards for flaw detection could well be a fertile field of

investigation. Over the years, users

and manufacturers of eddy current

equipment have come up with a wide

variety of methods for calibrating

equipment to do flaw detection. These

have included round file notches, drilled

holes, electrical-discharge machined

(EDM) notches, machined rectangular

notches, and machined "V" notches.

While any of these can be used to establish a repeatable machine set-up level,

there would seem to be considerable

question about how they relate to one another and to real flaws. In physical

47
appearance, an EDM notch, especially a narrow one, superficially appears to be

a better simulator of a natural crack

than does a drilled hole. But how

significant is a physical similarity? A question that has been raised and never

fully answered is whether an artificial

standard can truly simulate a real

crack. Cracks are the result of stresses

within the material, and there is

usually some residual stress field

remaining that can modify the eddy

current response. How true is this

idea, and is it true for all types of

cracks in all materials? These are only

a few of the dozens of questions that

come to mind when considering this

particular aspect of eddy current

standards.

The eddy current inspection method

has become a major tool for the resolution

of problems not amenable to solution by

other nondestructive testing methods.

Its application in research and development programs and in manufacturing and

service applications is essential for

any well-rounded nondestructive testing

program. Advancing gas turbine technology

will require continuing development of

inspection techniques, equipment, and

standards.

Comment (Mr. Weismantel): I think one of our problems in the NDE area is that we

keep trying to make notches to represent

the flaw we are trying to find. The

purpose of an artificial defect is to make a reproducible condition so some facility on the other side of the

country can set up that particular

condition, and essentially work to the same sensitivity or a similar sensitivity. I do not think that we will ever get to the point where we will be

able to use a notch to represent the

flaw you are looking at, because flaws

vary so much. The purpose of the

standard is not to represent the flaw,

but to bring you to a point where you

can find the flaws that you have shown.

This you can do under certain conditions.

Comment (Mr. Brown): I am not sure whether to make these comments now or later when we get into the nuclear area. But, I wanted to pass on some experiences I have had with EDM notches. They

are not all alike. I had some EDM notches made and the person who was making them brought the electrode down

on the tube and moved it back and forth

a little bit so that the end of the

electrode would be curved just like the

tube, and then he moved it over and made

the notch. He said, "I do that all the

time when I am making ultrasonic notches,

and it does not cause them any trouble."

I found tremendous signals from places

where he had burnished the electrode. If

you are not aware of this effect you may I think the signals are coming from the

notch of the standard; they are not,

they are really coming from the burnished

place. If you use EDM notches as standards

for small notches, be careful when they

do not appear to be consistent. It apparently has something to do with the

conditions under which the notches were

made, the oil, temperature, etc., and do not let them burnish the electrodes.

Question : How do you validate your

measurements for the case of the multilayer laminate you talked about?

Answer (Mr. Betz): As I said, we did it

as a feasibility study. What we did to get the data that we obtained was have our people make us a maximum and minimum

coating on the base material, and then a maximum and minimum second layer to get

the proper combinations, and then we went to a nonconducti ve shim stock for

the third layer.

48
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gai thersburg , MD, November 3-4, 1977. Issued

January 1981.

EDDY CURRENT EXAMINATION IN THE NUCLEAR INDUSTRY

Allen E. Wehrmei ster

Babcock & Wilcox Company

Lynchburg Research Center

Lynchburg, VA 24505

This paper discusses eddy current

testing in the nuclear industry. Almost

all eddy current inspection at Babcock &

Wilcox (B&W) is for tubing; tubing specifically for nuclear steam generators. In- spection is performed during tube manu- facture and after installation in steam

generators.

An automatic shop tube inspection

station is shown in figure 1. Straight

lengths of tubing are inspected for

anomalies formed during the manufacturing

process. The types of tube anomalies are predictable and readily detected with the

eddy current method. The consistency of

daily shop operation yields a highly

reliable test system.

Tubes installed in steam generators

are subjected to a more complicated test

environment. Field inspection of these

tubes is the primary purpose for my

participation in this NBS workshop. Two

types of steam generator concepts are widely used, the recirculating steam

generator (RSG) or U-bend generator and

the B&W designed once- through steam

generator (OTSG). The OTSG has all

straight tubes, no U-bends. The test

problems encountered in these generators

can be similar. I will discuss some of

the inspection problems encountered, how

some have been overcome, and others that

still require a solution. These problems

have a significant adverse impact on the

reliability of eddy current examinations

of install pri tuhpc

Figure 1. Automatic eddy current inspection station for steam generator tubes.

49
What is an OTSG test environment?

The OTSG is about 60 feet high and con- tains 15,500 Inconel 600 tubes (see

fig. 2). Superheated water (referred to

as primary side water) enters the top of

the generator and exits at the bottom.

The secondary side water (on the outside

of the tube) enters at the bottom and con- verts to steam, exiting at the top. There

are 15 tube support plates located along

the length of the generator. The supports

are made of 1-1/2 inch thick carbon steel

plate. Each tube passes through each

support.

,CI[ll!IC OIKtil

IF MCE TIMItl

STF.il SdEIITII

15 500 lakes

1(4 Kills [lllll lll|lk|

140.000 Ft. 2 SlrtlM liu il Tikis S till ll 1 Flllklll Flllll

Figure 2. Schematic of Once Through

Steam Generator (OTSG). Vertical

tubes are supported by 15 horizontal

plates. Two OTSG's are in each

nuclear steam system.

The tube is inspected for general

wall thinning as a probe is driven into

the steam generator. Examination for discrete flaws is made while the probe is

being withdrawn. The probe drive and ma- nipulator system are sketched in figure 3. The eddy current signals are recorded on magnetic tape and on a strip chart for

post analysis. B&W uses a test frequency

which produces about one standard depth of

eddy current penetration in the tube wall.

Typical signals at this test frequency are displayed as shown in figure 4 (from artificial flaws). A range of signal orientations (phase angles) are used to establish flaw through-wall penetration.

The data are taken to a data analysis center for post-test review. When

"flaw- like" signals are detected, the

questionable region is examined again at

other test frequencies for more information. Multi-frequency examination is used

to validate anomalies and perform flaw

characterization. Leaks between the primary and secondary sides are of primary

concern, but eddy current examinations do not detect leaks. Leaking tubes are identified with hydrostatic tests. Eddy cur- rent examination detects tube anomalies

which may or may not have leak potential.

Any phenomenon that interferes with

the flaw signal shape or orientation

affects the ability of an analyzer to interpret the data. The support plate produces an eddy current signal pattern like

a horizontal figure eight. The tube

region within ± 1/2 inch of each edge of a support plate is subject to the possibility of a flaw signal mixing with the

tube support signal. For each tube support plate, therefore, about 2 inches of

tube is masked by an interfering tube sup- port plate signal. That represents 32

inches out of 56 feet of tubing; and as it

turns out, these areas are the most critical regions in the generator. The instrument on the right of figure 5 is a com- puter system that was designed to eliminate the effects of the tube support signals during analysis. This signal processing makes the signal look as though it were from free and clear tubing.

Figure 6 illustrates the computer

signal processing concept. When a differential eddy current coil system detects

a crack in free and clear tubing, a classical flaw signal 1 is generated. When

one edge of a support plate is detected,

one half of the horizontal figure-eight

pattern is generated 2 . When a crack is at the edge of a support plate, a distorted tube support signal (or a distorted

flaw signal) is generated 3 . Subtracting

the tube support signal from the distorted

signal results in a classical flaw signal

that can be interpreted.

Figures 7 and 8 are examples of

actual inspection signals before and after

signal processing. The resultant indications are classical flaw signals from

the outer surface of the tube. This analysis is not clear from the distorted sup- port plate signal deviation alone. We

must analyze that deviation and judge its

significance. When support plate signals

are distorted, they represent a deviation

from normal, something detected. Unless

the signal deviation is studied and its cause established, we do not really know

what has been detected.

50
MASTER

Pivot Locator

Figure 3. Master/Slave probe manipulator concept. Probe position is

verified with a television system prior to inserting probe in tube.

The Master template and eddy current instruments are remote from the

OTSG.

63.5%

10.8%

VERTICAL HORIZONTAL t

STRIP CHART

Figure 4. Oscilloscope and strip chart tracings of artificial anomalies

10.8%, 36.5%, 63.5%, and 100% of the tube wall thickness.

51
Figure 5. The first Computer Eddy Current Analyzer (CECA-1) shown during

field post-analysis.

SUPPORT PLATE

CD

CRACK

EDDY CURRENT PROBE

"N CRACK AT EDGE

OF SUPPORT PLATE (D CD CD

V Ho \f

Figure 6. Illustration of the computer signal processing concept.

52
DISTORTED SUPPORT PLATE SIGNAL

1

ANALYZED SIGNAL 40-50% O.D.

IISTIITEI UPPER TIRE SHEET SIGNAL

NORMAL UIS SIGNAL (REFERENCE)

ANALYZED INDICATION

Figure 7. A distorted tube support plate

signal and the resultant flaw signal

after subtracting a good support plate

reference signal. Tracings of actual

field data.

For example, flaws are not always the cause of distortion or signal deviations.

The distorted signal in figure 9 produced

a "chatter" indication when analyzed with

the computer system. "Chatter" or ID ripples are produced during tube manufacture. It is not considered detrimental, unless its signals mask all flaw

signals. To eliminate ID chatter signals

is to improve analysis.

Figure 8. A distorted upper tube sheet

signal, a reference tube sheet signal,

and the processed resultant signal.

Tracings of actual field data.

Figure 10 shows what the effects of

cold working or residual stress have on a flaw. Forty percent and sixty percent EDM notches were cold worked (rubbed with the

shaft of a screw driver) in the laboratory. In each sample, the phase information was distorted, yielding incorrect

information about flaw depth. The distorted signals made the flaws appear

deeper. A dent and a 100 percent through

1st SUPPORT PLATE DISTORTED SI6NAL

i

mm cue warn 1

I

60% em nitch

V, ii. LING

III COLD WORK V / ^

f

WITH C8L0 WORK / NO COLD WORK

40% EDM NOTCH

%n. LONG

ANALYZED SIGNAL "CHATTER"

DING X

100% HOLE

v 100% HOLE PLUS « DING

LOOKS LIKE 70% FLAW

Figure 9. A distorted support plate

signal and the processed non-flaw

resultant signal. Tracings of actual

field data.

Figure 10. Experimental signal tracings,

illustrating the effects of "cold

working" on signal shape and

orientation.

53
wall hole, however, appeared like a shallow flaw. These are signal analysis (flaw

characterization) problems that develop

because of external influences on real

flaws. What other mechanisms are at work?

The signal shown in figure 11 was monitored during repeated in-service inspection. Analysis indicated that a flaw

was growing, and that it was deep. When

the tube was removed from service, the

eddy current indication was analyzed as shallow. Destructive tests confirmed a shallow flaw. The "effect" (stress?) that

caused the distorted information disappeared when the tube was removed from service. The cause of the distortion, or the

"effect" producing incorrect analysis, has not been determined. The development of a technique to eliminate the influence of

this "effect" is required.

It is not held by the tube support plates,

it is only confined to a region.

Question (Mr. Weismantel): Could you give

me some idea of the sizes of the defects

you are seeking and what the sensitivity

level is?

Answer (Mr. Wehrmeister): We look for 20

percent throughwall indications in accordance with Reg. Guide 1.83. B&W tubing has

a 0.037 inch wall.

Question (Mr. Weismantel): What is the

length of that 20 percent throughwall?

Answer (Mr. Wehrmeister): No length is

specified.

Question (Mr. Weismantel): Regardless of

whether it is 20 thousandths long or 100

thousandths or ten inches?

I.S.t. DATA JUNE. 1977

DISTORTED 15th S.P.

Answer (Mr. Wehrmeister): That is right.

Question (Mr. Weismantel): The interference you obtain from support plates, I gather you tried higher frequencies to

null out that interference?

I.S.I. DATA IUNE, 1977

DISTORTED 15th SUPPORT PLATE

ANALYZED FLAW SIGNAL I EDDY CURRENT SIGNAL FROM REMOVED TUBE

IN NIT CELL AUGUST. 1977

Figure 11. Tracings of actual field data

illustrating the effect of "other

factors" on signal shape and

orientation.

Di scussion

Question (Mr. Ammirato): Are you able to inspect near the tube sheet?

Answer (Mr. Wehrmeister): Yes, the tube

sheet and the tube support edges are similar; you get the same kind of response.

Each can be analyzed with the computer

system.

Question (Mr. Ammirato): How is the tube

sealed in the support plate, compared to the tube sheets?

Answer (Mr. Wehrmeister): The tube

welded and rolled into the tube sheets.

is

Answer (Mr. Wehrmeister): Higher frequencies do not null it out; they lower

the sensitivity to outer tube surface

anomal ies.

Question (Mr. Weismantel): You would not

see the support if you went to a higher

frequency. Would that give you an adequate inspection?

Answer (Mr. Wehrmeister): No, shallow 0D

discontinuity signals would be smaller and

approach a horizontal position, thereby

making detection difficult. Higher frequency is used when we are looking for

phase angle relationships to establish

depth.

Question (Mr. Weismantel): Are your sup- port plates carbon steel or stainless?

Answer (Mr. Wehrmeister): Carbon steel.

Question (Mr. Titland): Do you calibrate

your computer on the support plates inside

the steam generator, or on a model?

Answer (Mr. Wehrmeister): We use the support plates in the generator.

Question (Mr. Brown): Do you use one sup- port plate chosen because you like the

looks of it, or do you take several and

average them.

54
Answer (Mr. Wehrmeister): We use those

that appear most consistent, we use signals from a previous inspection.

Question (Dr. McMaster): Do you have

much evidence of stress corrosion signals

in these tests?

Answer (Mr. Wehrmeister): We have not established the cause of some signals, but

none to date resemble what you might ex- pect from stress corrosion.

National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg , MD, November 3-4, 1977. Issued

January 1981

.

EDDY CURRENT INSPECTION SYSTEMS FOR STEAM

GENERATOR TUBING IN NUCLEAR POWER PLANTS

Clyde J. Denton

Zetec, Inc. Issaquah, WA 98027

1. Introduction

Eddy current inspection of steam

generator tubing in commercial nuclear

power plants has evolved from a simple

manual effort to test two tubes during

1970, to completely automated systems

inspecting thousands of tubes today.

Although improvements have been

made in the recording and interpretation

of data, as well as in mechanical fixturing, the basic eddy current test is

still performed in the same way.

The following is a brief description of the eddy current test technique.

An alternating voltage is impressed

across two test coils. The magnetic

field developed by current flow in the

test coils causes eddy currents to flow

in the tube wall. The corresponding

magnetic field caused by eddy current

flow in the tube wall is out of phase

with the field developed by the current

in the test coil. Since, these fields

tend to cancel one another, the coil

voltage is decreased in proportion to

the magnitude of eddy current flow in

the test piece. The magnitude of the

eddy currents in the test piece, thus

the coil voltage, is dependent on the

electrical properties of the tube being

tested. The electrical properties which

affect the flow of eddy currents are permeability and conductivity. In non- magnetic materials, such as Inconel and

300 series stainless steel, conductivity

is usually the only significant variable.

When the effective conductivity de- creases due to a discontinuity in the

tube wall, the coil voltage increases in

direct relationship with the effective

conductivity change. Thus, the amount

of increase in coil voltage is related

to the size of the discontinuity. The

coil voltage is sinusoidal; thus, it can

be described with a single vector having

magnitude and phase. The Zetec eddy

current system provides a method to read out and record the two quadrature

components of the test coil voltage

vector.

2. Discussion

The system employed to inspect

steam generators uses eddy currents as the probing media to measure variations

in the conductivity of the tube wall

being tested.

An alternating voltage is impressed

across two test coils. The magnetic

field developed by current flow in the

test coils causes eddy currents to flow

in the tube wall. The corresponding

magnetic field caused by eddy current

flow in the tube wall is out of phase

with the field developed by the current

in the test coils. .Since these fields

tend to cancel one another, the coil

voltage is decreased and phase shifted

in proportion to the magnitude of eddy

currents in the test piece; thus, the

coil voltage is dependent on the electrical properties of the tube being

tested. The electrical properties which

affect the flow of eddy currents are permeability and conductivity. In non- magnetic materials, such as Inconel and

300 series stainless steel, conductivity

is usually the only significant variable. When the conductivity decreases

due to a discontinuity in the tube wall,

the coil voltage increases and phase

shifts in direct relationship with the

depth and volume of the conductivity

change. Thus, the amount of increase in the coil voltage and the phase change is related to the size of the discontinuity.

The coil voltage is sinusoidal;

thus, it can be described with a single

vector having magnitude and phase. The

eddy current test system used in steam

generator inspection provides a method

57
for reading out the two quadrature

components of the test coil voltage

vector.

The two test coils are electrically

connected in opposite legs of the

balancing network in the eddy current

instrument. Thus, the tube is being

inspected by the differential technique.

The differential technique decreases the

effects of probe motion, temperature

variations, and geometry differences.

However, changes in nominal wall thickness are not detected.

The electronic portion of Zetec's

eddy current system contains five separate instruments. The main instrument

is a Zetec/Automation Industries EM-3300

Eddy Current Tester. The EM-3300 has a continuously variable frequency from 1 kHz to 2.5 MHz with a digital readout to indicate the operating frequency. The

readout is accomplished on an X-Y memory

oscilloscope which is an integral part

of the EM-3300. The instrument has X-Y outputs of plus or minus 8 volts and a frequency response of DC to 100 Hz.

The output of the EM-3300 is con- nected to a Zetec FM-2300S Two-Channel

Magnetic Tape Recorder. The tape re- corder also has input and output capabilities of plus or minus 8 volts and DC

to 100 Hz frequency response. In addition to recording the X-Y channels, the

tape recorder has a microphone to allow

tape recording tube identification and

other pertinent data. The circuits in

the recorder are designed to allow voice

insertion and retraction without interaction with the test data.

The output of the FM-2300S is con- nected to the input of a Two-Channel

Strip Chart Recorder. The strip chart

recorder has a frequency response range

from DC to 100 Hz, and it is capable of

displaying a voltage input of plus or minus 8 volts. The strip chart recorder

provides two functions. First, it

provides a permanent record which can be

scanned rapidly for initial inspection

results. Secondly, since it monitors

the output of the magnetic recording, it assures that the recording equipment is

functioning properly.

The fourth instrument is a Zetec

Model I Communications Amplifier which

allows voice contact between four sta- tions with variable inputs and outputs

for all stations. The amplifier contains high and low

normal plant noise.

filters to decrease

The fifth instrument is used to assist in data analysis and will be

discussed at length later in this

presentation.

The eddy current test system is normally used in conjunction with a mechanical system which positions the

probe over the correct tube and then

inserts and withdraws the probe. The

insertion rate is approximately two feet

per second and the withdrawal rate is one foot per second. The inspection is

performed during the retraction of the

probe.

When the probe is inserted the

proper distance, the tube number is

written on the strip chart and the voice

entry is made on the magnetic tape, then

the probe is retracted while the re- cording systems are operating.

When the magnetic tape is com- pleted, the tape and its associated

strip chart records are taken to a remote location where they are analyzed

by an ASNT-TC-1A Level IIA qualified

interpreter.

The equipment used to analyze data

consists of a tape recorder identical to the one used to record the data, and a vector analyzer which more realistically

should be called an electronic protractor. The "analyzer" provides a rapid means of measuring the phase angle

and amplitude of signals.

The basis for phase analysis eddy

current testing can be simplified and

explained as follows. Given four

concentric tightly fitting tubes as shown in figure 1, and starting with the

I 1 TUBE

/ 4 2 TUBES

,' //3 TUBES

I//'/"> TUBES

PHASE I AMPLITUDE

Figure 1. Phase relationships.

58
probe in air, first the air vector is

obtained. When the probe is inserted in

the smallest diameter tube, eddy currents

flow in the tube wall with a resulting

magnetic field. The resultant coil

voltage vector is decreased in amplitude

and phase shifted. As the second tube

is slipped over the probe area, the

vector amplitude is further decreased

and phase shifted. The current flowing

in the second tube is a function of the

magnetic field from the coil and the

magnetic field associated with the

current flow in the first tube. This

process continues for each tube with the

current flow in each tube dependent on the current flow in the adjacent tubes.

The eddy currents are not affected (in a nondefective tube) by the laminar type

tube to tube interface. Thus, this

example can be expanded to include eddy

current flow in a solid tube wall. The

current flowing in any circumferential

tube segment has its own distinctive

phase and magnitude. The exact phase

and magnitude at any point in the tube

wall is dependent on the test frequency

and the conductivity of the tube being

tested. The eddy current test system's

function is to detect and record variations in the magnitude and pattern of

eddy current flow in the tube wall.

When a differential probe is passed

through a tube with a defect, the signal

is formed as in figure 2. Point 1 of

figure 2

GOOD TUBE DIFFERENTIAL COILS

" 100% DEFECT SIGNAL FORMATION

probe was a differential bobbin type and

the two defects not penetrating through

the wall are on the outside surface of

the tube.

Figure 4 is essentially the same as Figure 3 except additional defects are shown and the optimum frequency, wall

thickness, and conductivity are used.

100

Figure 3. Signal phase angle comparisons

at three frequencies.

PHASE ANGLES AT 400KHz

7>fc OD X .050 INCONEL TUBE

DIFFERENTIAL COILS

Figure 2. Signal formation.

is the signal from a good tube, point 2 shows the first coil approaching the

defect, point 3 shows the coil directly

centered in the defect, point 4 shows

the first coil leaving the defect and

the second coil entering the defect, and

point 5 shows the completion of the

signal

.

Figure 3 shows three defects tested

at three different frequencies. The

Figure 4. Actual phase angles at optimum

test frequency.

Taking the data from figure 3 and

plotting a calibration curve of percent

penetration of the tube wall versus

signal phase angle results in the data

presented in figure 5.

The eddy current test sytem has been

shown to exhibit a long term two sigma

measurement error of plus or minus 5 percent under actual field conditions.

59
Plotting this information versus the

calibration curves in figure 5 results

in the measurement error curves shown in

figure 6.

40 60 80 100 120 140 160 180 200 220 DEGREES

Figure 5. Calibration curves for three

frequencies.

20

i 100 KHz

15 - 200 KHz

+ 10 - i.00 KHz

5

- 0

-

S

10

IS - MEASUREMENT ERROR

20 -

20'fc 50% 100% OF WALL

Figure 6. Measurement error comparisons

at three frequencies.

Note that the test sensitivity

shown in figure 5 indicates more sen- sitivity at 100 kHz than at 400 kHz, but

the measurement error curve shows twice

as much error at 100 kHz. This is one

of the considerations which determined

the selection of 400 kHz for flaw

detection in 7/8 inch and 3/4 inch x .050 inch wall Inconel 600.

The mechanical portion of the Zetec

eddy current test system varies to accommodate the conditions imposed by

the designs of the various steam generator vendors.

Basically, all of the systems

function as follows. A template with

tube number identification is temporarily

installed in the steam generator. A rotatable circular fixture with a minimum

of two independent motions is installed

over the template. The fixture operator

positions the probe guide tube and its

associated light and TV camera over the

tube to be inspected. The probe/pusher

puller mechanism is used to insert and

retract the probe. Test speeds of over

100 tubes per hour are achievable when

the tube test length is short. Thus, it

is obvious that fixture positioning time

is relatively short. The complete data

station and fixture control center can

be operated up to 150 feet from the

steam generator, although shorter

distances are recommended.

Discussion

Question (Dr. Mc Master): You did not

mention the Russians. Are they using

your services?

Answer (Mr. Denton): The reactors that

we have been involved with are in

Finland. What they have done is copied

the Hanford tube sheets; so it is essen- tially the same system. The Hanford

tube sheet has a dual pipe going in and

out. The inlets are on top, outlets on the bottom. The Russians merely took

that and made it two different tube

sheets, with an inlet and an outlet, and

the tubes still go both ways.

Question : On the B&W steam generator,

do you use a template?

Answer (Mr. Denton): Yes. There was a template. I do not know if it is NRC or ASTM code-- somewhere in the system it

says you have to positively identify the

tube. That sounds great when you write

it, but realistically when you are 100

feet away, to check this thing, you have

to put on two pairs of coveralls, boots,

gloves, etc., go inside and say, that is

the right tube, all right. So the

template and TV system eliminate that. Even if you have a system that has dials

on it and it does not really require a template, you may still put it in just

to satisfy the positive ID of the tube.

Question (Dr. Green): Doesn't one person often take the data while a second person analyzes it?

Answer (Mr. Denton): Yes. The data is

stored on magnetic and paper tape and no analysis is done on the job at all.

There are many reasons why we do it this

way.

Question : Are there any changes in the

characteristics of the probe due to the

radioactive environment?

Answer (Mr. Denton): No.

60
Comment (Mr. Wehrmeister): Water in the

generator tube also does not affect the

test. We inspect generators prior to draining in what is called the critical

path. It costs upward of a quarter of a million dollars every day a generator is

down; so you want to complete the

inspection as quickly as possible. So we do inspect them while they are still

full of water.
b
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gai thersburg , MO, November 3-4, 1977. Issued

January 1981.

USE OF ROUND ROBIN TESTS TO DETERMINE

EDDY CURRENT SYSTEM PERFORMANCE

E. R. Reinhart 1 In-Service Inspection Incorporated

333 Victory Avenue

South San Francisco 94080

1. Introduction

The operational availability of a number of Pressurized Water Reactors

(PWRs) has been reduced by the recent discovery of deformation and cracking in

steam generator (SG) tubing in several

operating reactors [1,2] 2. The more

severe deformation is known as denting

and occurs in the area of the tube

support plates. In-service inspection,

during periods of reactor shutdown, is

presently used to detect and analyze this

problem. To satisfy regulatory requirements for in-service inspection of steam

generators, the only inspection method

presently used and accepted is eddy cur- rent testing (ET) [3] . For this inspection, differential coil bobbin type eddy

current probes are inserted in the inside

diameter (ID) of the primary side of the

steam generator and drawn through the

length of the steam generator. The

present eddy current systems and techniques were evolved from technology developed during the early 1 960 1 5 [4]. In- service inspection experience (training

of inspectors, analysis of data, etc.)

was primarily derived from the involvement of various groups with the Nuclear

Navy. In the past, this test has been

very successful in detecting such problems as wastage and corrosion in straight

sections of steam generator tubing [5].

However, the recent occurrences of denting

in the tube support area provide the inspector with complex eddy current signals

that may mask flaws. Denting and "ovalization" of tubing also restrict access by

the inspection probe. Questions have also

been raised regarding the capability of

the existing eddy current methods to

determine, in subsequent inspections, the

extent of slow flaw growth to the degree

necessary for judging the effect of re- medial SG activities (change to all volatile treatment, etc.).

In response to the obvious need for

improved NDE technology, considerable

activity is being funded in NDE systems

and development for SG inspection by EPRI,

government agencies inspection groups,

nuclear system steam suppliers (NSSS),

and foreign groups [6,7,8]. Multifrequency ET, new ET probes, and ultrasonic

systems are all in various stages of

development. In light of the present SG

problems, the utilities need to sort this NDE activity into the categories of ex- pected near-term improvement (within six

months) and mid-term improvements (within

12 months). The near- term improvements

should have the potential to improve inspection performance for the next series

of major in-service inspections (winter

1977) and for units that will be cleaned

or where water treatment will be changed.

The near-term improvements would therefore

reflect system changes that are now ready for field use but require qualification. In the area of mid-term

improvements, systems technology should

be identified that is amenable to accelerated effort for incorporation

into systems that can be used for fall

1978 in-service inspection. In addition,

goals for long-term R&D activity should

be defined.

To address these needs, as well as to define a baseline for existing SG

formerly with Electric Power Research Institute, Palo Alto, CA 94303.

2 Figures in brackets indicate the literature references at the end of this paper.

63
inspection capability, EPRI recently

initiated a technical round- robin program.

Conventional NDE methods and advanced

multi frequency ET systems were evaluated.

A panel of in-service inspection specialists and theoretical NDE consultants

observed and participated in the roundrobin evaluations.

The results of this program will be

used by EPRI in two areas.

(1) Plan long-range R&D projects

for the EPRI Nuclear Division. This study

will establish the performance level of

present inspection systems and point out

areas where long-range R&D activities

should be conducted. Most of the shortand mid-term development effort in this area will be conducted by EPRI 1 s newly

established Steam Generator Project Office

described below.

(2) Define NDE performance goals

for the Steam Generator Project Office.

The Steam Generator Project Office has

been established by EPRI and member

utilities to rapidly develop technology

to alleviate serious losses in PWR plant

availability caused by the previouslymentioned problems associated with steam

generators. The Steam Generator Project

office has identified NDE development

effort as a key item in its plan for

improved availability; it will therefore

use the results of the technical planning

study to focus attention on the areas that

have the most potential for achieving

near-term improvement.

Details of planning and conducting

the study are presented in the following

sections.

2. Planning the Program

Determining the nature of present

SG NDE inspection problems, determining

the performance of present and developing NDE systems related to those

problems, and planning remedial action were considered the major objectives of

initial EPRI activity in this area. From

a review of past work in studying NDE system performance, conducted by EPRI and

others, the following steps were taken

in planning an initial study [9,10,11].

2.1 Definition of problem.

This first step in planning the

study involved a compilation and study

of available reports on the subjects of

SG tubing flaws and in-service inspection. Reports that were of particular

value in planning the program are listed

as references 12 through 17. These

reports gave a fairly good assessment of

the location, nature, and frequency of

defects found in present pressurized

water reactor (PWR) SG designs. Many of

these reports were obtained from a literature survey conducted by Battel le Columbus Laboratories for this study.

Although there was considerable

information on several types of SG

problems, these reports lacked detailed

information on the denting problem. For

a better definition of this problem, an NDE specialist meeting was therefore held

on February 24, 1977, at the offices of

EPRI. From the results of this meeting,

a better idea of the nature of the denting

problem was obtained, along with considerable information to aid in planning an NDE performance evaluation study. Selecting the type and nature of the study is

discussed in the next step.

2.2 Definition of study.

From the results of the NDE specialist meeting, the literature survey,

and several additional communications,

an EPRI Technical Planning Study (TPS77-709) was selected as the vehicle for

conducting further effort in this area. Technical planning studies are conducted

by EPRI to support research and development planning for the engineering and

economic feasibility of proposed technological development and/or hardware

options. Such studies permit identification of the most promising options and

the major technological issues which must

be resolved before the initiation of a comprehensive research program. The

technical planning study approach was also selected since this represents one

of the most expedient EPRI methods

(minimal contractor negotiation time,

streamlined review and approval process,

etc.) for responding to studying near- term utility problems. Major objectives

of this study were defined as:

(a) First, the overall baseline

performance of present NDE systems (including the operators) in response to a variety of defect types should be determined.

This baseline would establish the nature

and extent of future R&D activities.

(b) Second, the performance of

several new inspection methods, tech64
niques, and equipment, should be evaluated

j to determine their potential for solving

present NDE problems. Both field prototype as well as laboratory methods should

I be evaluated.

(c) Third, the study should be

initiated and completed as soon as possible

in order to transmit the information to the EPRI Steam Generator Project Office

I and other interested EPRI Nuclear Departi ments for use in planning comprehensive

J R&D programs.

2.3 Organization of the study.

Since the nature of the inspection

problem was recognized as being very com- plex, and since EPRI needed to rapidly

obtain as much comprehensive information

as possible, a technical round-robin

program, aided by theoretical and applied

NDE specialists, was selected as the

basis for the study. It was felt that

the data from simulated in-service inspections, when combined with the analysis

and observations of an expert review

panel, would provide considerable insight

into the various parameters affecting

inspection system performance.

2.4 Details of the study.

This study incorporated the following

detai Is:

A. NDE Evaluation Panel. Under the

direction of an EPRI Project Manager, a six-man NDE technology evaluation panel was used to access performance of the

various NDE systems. The panel was com- posed of one NDE inspection specialist

from the following Nuclear Steam System

Suppliers: Babcock & Wilcox, Combustion

Engineering, and Westinghouse. The re- maining three members were selected from

the following independent groups: Battel le

Columbus Laboratories, EPRI, and Southwest

Research Institute.

Battel le and EPRI are independent

research laboratories whereas Southwest

Research represents an independent inservice inspection group. The above team,

composed of both NSSS suppliers and independent laboratory representatives, was formed to lend credibility and objectivity

to the project results. The above groups

also supplied examples of defective tubing

and aided in developing a realistic test

program.

Each nondestructive testing system

was evaluated by this panel in the

following manner:

(1) General impressions. Prior to laboratory tests, details of the

system were described by the

system suppl ier.

(2) Scan of known defects. The

panel was allowed to review the

system in operation and review

such details as data analysis,

etc.

(3) Scan of unknown defects. Data were then taken using a mockup

containing a series of simulated

defective tubing.

(4) Summary of results. Based on the results of (1), (2), and (3)

above, each panel member

submitted his conclusions to EPRI regarding the performance

of the NDE system under evaluation.

A mockup containing examples of defective tubing was essential to conducting

the study and is described in the following section.

B. A key element in any study of

in-service inspection performance is simulation of the inspection environment that

the NDE system "sees." In this respect, a realistic mockup is essential. Since this

study was aimed at determing current SG

inspection performance as used in the

nuclear industry, all three NSSS SG designs were considered. Looking at the

present three SG designs depicted in

figures 1, 2, and 3, the task of designing

a realistic mockup initially appears monumental. This would be true unless one considers that the inspection systems to be evaluated in this study only have access to the tube sheet and inside sur- face of the heat transfer tubing. With

respect to possible mockup configurations,

table 1 presents various configurations

that could be used for NDE performance

studies or systems development. The

mockup design selected for this study was configuration 3. An air transportable

mockup was designed since several of the

systems that were to be evaluated were in the laboratory or prototype stage of development, and transport of these systems

from the laboratory was not feasible.

Transporting the mockup to the various NDE development laboratories was also optimun

from a scheduling and economics standpoint.

65
Figure 1. Example of U-bend steam generator design (similar to Calvert Cliffs

# 1).

Steam Outlet to Turbine Generator

Upper Shell

Tube Bundle Tube Support

Lower Shell

Feedwater

Inlet

Tube Plate Primary Coolant

Outlet Channel Head

Primary Coolant

Inlet

36 in. (~91.4 cm)

Primary Inlet Nozzle

Auxiliary Feedwater

Inlet

Tubes

24 in. <~61.0 cm) Steam Outlet Nozzle (2)

Feedwater Spray

Nozzles (32)

Figure 2. Second example of U-bend steam

generator design (similar to Surry # 1).

28 in. (~71.1 cm)

Primary Outlet Nozzle (2)

Figure 3. Once-through steam generator

design (similar to OCONEE #1).

Essential features of the mockup are shown in figures 4, 5, 6, and 7. The

mockup was designed and built by Battelle

Columbus Laboratories within one week of

contract initiation. The mockup proved

to be easy to transport and assemble at the test site, and could be used to eval- uate a large number of SG tubing configurations, including tube supports and U- bends. The next section describes the

samples used with the mockup to test the

various systems.

C. Test Samples. Although the study

addressed NDE problems associated with all

existing NSSS steam generator designs, one tubing size for all the test specimens was selected to simplify the logistics of the

program. For the same reason, samples were all 7/8 inch nominal OD (.050 wall)

Inconel 600 steam generator tubing of a configuration typical to several SG

designs, including the Westinghouse series

51 PWR steam generators. The tubing

samples were either supplied by members

of the NDE evaluation panel from existing

test samples or fabricated specifically

for the EPRI study. The samples included

66
Table 1. Steam Generator Mockup Configurations.

Mockup

Configuration Purpose Key Elements in Design

1 Develop and evaluate

NDE for inspection

of the secondary

side of the SG (OD

inspection).

Complex access must be simulated

with simulated tube bundles,

outside surface of SG, tube

supports, etc. This is probably

a fixed site design. The space

required depends on the SG design;

however, it could be of limited

height for inspection at only one

level

.

2 Develop and evaluate

total NDE system for

inspection of SG

tubing from the

primary side.

To evaluate total NDE system,

including remote positioners,

etc. , total simulation of tube

sheet geometry and distances is

required. Realistic tubing lengths

and U-bend geometry is also required.

This design is probably fixed

site and the overall height could

be considerable (>50ft). (see

Reference 5).

3 Develop and evaluate

basic inspection

system probe and

instrumentation

performance under

simulated dynamic

inspection.

Inspection environment could be

simulated with single or limited

number of tubes. Tube supports

and vertical tube sheets are simulated with sleeves over the

tubing. This design can be fixed

or air transportable

conditions.

4 Develop and evaluate

NDE system or components unoei

laboratory

conditions, with

laboratory controlled

probe motion.

Simple tubing holder with probe

motion controlled by simple probe

Drive mecnan i sm. i uoe u i stances

can be as small as 6 inches (see

Reference 10).

specimens loaned to the program for an ongoing Battelle Col umbus/Brookhaven

National Laboratory program through the

courtesy of Dr. John Weeks of Brookhaven.

The nature of the various samples were:

Notched Samples . These samples used

electrodi scharge-machined (EDM)

notches to simulate narrow crack-like

defects (fig. 8). EDM notches ranged

in width from 0.005 in to 0.009 in. The flaws were machined at various

depths, lengths, and orientations

(axial, circumferential, and at 45°

to the tube axis). Two samples were

also machined with the same flaw, one with work hardening, one without.

Figure 4. Air transportable steam generator mockup as shipped.

67
Figure 5. Mockup during assembly.

Figure 6. Mockup during simulated SG

inspection as viewed by evaluation

panel

.

Figure 7. Mockup as seen by inspectors

during simulated SG inspection, tubes

on opposite side. I

Tapered EDM CD. Notch

Max. Depth—60% of Wall

Section A-A 23"

Figure 8. Typical configuration of axial

notch specimen.

68
Wastage Samples . Samples representing wastage type defects were obtained by grinding metal from the

outside surface to simulate largevolume wastage type flaws (low depth,

large surface area). Compound wastage, which is a large- volume (low

depth, large surface area) flaw,

combined with a low- volume (large

depth, small surface area) flaw, was also simulated in several specimens

since this condition has been seen in

service (fig. 9).

Dented Samples . The dented samples

consisted of the following configurations:

Minor dent . These samples con- tained tubing with circumferential dents ranging in diametrical restriction from 0.002 to 0.005 inch. EDM notches of

various depths and length and at various locations (center and

edge of dent) were machined in

these samples to study the capability of NDE for detecting and

sizing flaws in the presence of

dents. In all these specimens,

a carbon steel sleeve, simulating a tube support, was placed

over the dented section. Magnetite was also packed on the outside of the tube in the crevice

between the dented tube and the

tube support. Plastic end caps were glued to each end of the

simulated tube support to retain

the packed magnetite (figs. 10

and 11).

Major dent . These samples were similar to the minor dent speci- mens with the diametrical re- strictions increased to 0.050

inch.

Major dent with "oval ization "

.

The specimens listed in table

2 represent the dented tube con- figurations with the added com- plexity of diametrical "oval ization" in the region of the dent

(figs. 12 and 13). Since the

"ovalized" tube no longer permits simple slip on carbon steel

tube supports, the dented re- gions in these samples were wrapped with slit sections of

carbon steel (figs. 14 and 15). In these specimens, EDM notches

Figure 9. Specimen containing wastage

defect.

Plastic End

Closures Carbon Steel, Simulated Tube

Support and Packed FE3O4

7/8 Diameter Inconel 600 Tubing

Figure 10. Dent specimen, supplied by BCL.

Plastic Collar Joining Test Specimens

Aluminum Bracket — Holding Specimens

to Mockup

Figure 11. Dent specimen mounted in test

block.

69
were placed at the beginning

(one flaw), center (two flaws)

and end (one flaw) of the dent

section (figs. 13 and 14).

All of these samples were supplied by Zetec, Inc. Zetec also

supplied a calibration standard

of the type presently used in

the nuclear industry.

Pitting . These specimens contained

machined conical defects designed to simulate localized low volume pitting

of various depths and at various

locations.

Table 2. Dented and Oval i zed Test Samples

(Dimensions in Inches)

N0°' SIZE

0D I.D.

SHOULDER CENTER CENTER

MINOR

(A) MAJOR

(B) MINOR

(CJ MAJOR

(0) MINOR(E) MAJOR

( Fl

I r J

0.624 1.069 0.618 1.047 0.500 0.937

2 0.010 0.744 0.989 0.7Z4 0.968 0.620 0.862

3 0.010 0.859 0.891 0.039 0.871 0.735 0.766

4 0.015 0.634 1.063 0.618 1.031 0.500 0.921

5 0.015 0.7S4 0.931 0.724 0.951 0.620 0.845

6 0.015 0.359 0.391 0.839 0.851 0.735 0.747

7 0.020 0.644 1.057 0.670 1.015 0.500 0.905

8 0.020 0.764 0.973 0.724 0.933 0.620 0.828

9 0.020 0.375 0.875 0.839 0.839 0.735 0.735

Corrosion Samples . These samples

contained laboratory- induced intergranular cracks to simulate the

corrosion cracks occasionally reported in operating steam generators.

U-bend Samples . The U-bend samples

contained defects all starting at the

inside surface of the tubing. All

defects were EDM notches, and these were located at the tangent and apex areas of the tube, at the introdose

and extradose. To facilitate

fabrication, the EDM notches in these

samples were placed in the tubes

before the tubes were bent to their

final configuration. The inner rows of a series 51 SG were the only

U-bends simulated since the sharp

radius of curvature of the rows represented the most difficult access problems for U-tube inspection of

this SG design. The outer rows of

U- tubes, having a more gradual radius

of curvature, were considered to represent an inspection situation

similar to a straight section of

tubing and were therefore not used in

this study.

Tube Supports Drilled carbon steel

plates were slipped over the Inconel

tubing to simulate the influence of

tube supports on the eddy current

inspection (fig. 16). The influence

of the tube sheet was not simulated

in this study since problems in this area did not appear as severe as the

defect situations described above.

There was also insufficient detailed

information regarding problems in the

tube sheet area to allow simulation.

If warranted, this area may be

addressed in future studies.

Section X-X

Section Y-Y

Figure 12. Configuration of dented and

ovalized test samples.

270° 0° 90°

Defect

No. 1 ^

Defect -

No. 2 X

Raws

Figure 13. Location of defects in dented

and ovalized test samples correlates

with figure 12. 70
Figure 14. Tube specimen with oval i zed

dent and flaws, and split carbon steel

simulated tube support.

Figure 15. Oval i zed tube specimen with

simulated tube support as tested.

Figure 16. Tube specimen with slip-on

simulated tube support.

3. Tests of the NDE Systems

The following three SG NDE systems were evaluated:

(1) Zetec Incorporated. The basic

Zetec single frequency system represents

present industry state-of-the-art equipment and techniques. The system uses a bobbin type differential coil inspection

probe. The system is rugged and simple

but is highly operator dependent. A prototype system using a rotating ET probe was also evaluated. Tests of these systems were performed at the Zetec laboratories

in Issaquah, Washington.

(2) Holosonics/Intercontrole. This

system represents state-of-the-art French

field inspection technology and utilizes a multi frequency eddy current approach to improve detection and analysis of flaws in

the presence of extraneous signals (tube

supports, etc.). Final data analysis is

manual. Other components of the system

are also significantly different from

present U.S. field equipment. This system

was evaluated at the offices of Holosonics/

Intercontrole , Richland, Washington.

(3) Battelle Northwest Laboratories.

This is a multi-frequency system developed

from EPRI funding. The system utilizes a modified Zetec ET probe combined with a instrumentation system that acquires four

frequency data during inspection and automatically analyzes data from two of the

frequencies to eliminate extraneous signals from probe wobble, tube supports,

etc. This system was also evaluated at the Zetec laboratory in Issaquah,

Washington. This system was in the prototype development stage and this study was the first evaluation of the system

under simulated field inspection

conditions.

Each of the above nondestructive

testing systems was evaluated by the NDE panel in the following manner:

(1) General Impressions. Prior to laboratory tests, details of the system were described by the system

suppl i er.

(2) Scan of Known Defects. The panel was allowed to review the system in operation and review such details as data analysi s , etc.

(3) Scan of Unknown Defects. Data were

then taken using a mockup containing

a series of simulated defective

tubi ng.

71
(4) Summary of Results. Based on the

results of 1, 2, and 3 above, each

panel member submitted his conclusions to EPRI regarding the perfor- mance of the NDE system under evaluation.

A mockup containing examples of defective tubing was essential to conducting

the study and was previously described.

In general, the tests associated with

scanning the unknown defects progressed

from simple straight tubing configurations

to progressively more complex tubing and

flaw geometries. Straight sections of

tubing containing notches, pits, and wastage type flaws, and without tube supports, were tested first. Several of these tests were then repeated with tube supports

added to the test specimens. These supports were located near, at the edge, and

directly over the flaws. Placement was usually based on field experience with

real flaws.

After the straight sections, the various U-bend configurations were tested. A typical test configuration is shown in

figure 17. Following these tests, the

systems were evaluated using dented tubing

of various configurations. It should be

noted that the Zetec rotating probe system

was the only system capable of testing

moderate or extensive dents or dents with

an "ovalized" configuration. Zetec was also the only system possessing a probe

capable of testing U-tubes after passing

through a moderate or severe dent. For

this reason, fewer tests were run on the

other systems.

A number of tests were also conducted

to study the influence of probe design,

fill factor, test frequency, and gain on basic single frequency system performance.

The preliminary results of these

tests are discussed in a later section.

4. Data Analysis

A number of approaches can be taken

to analyze the data. The method presented

here is to consider two aspects of inspection system performance, the probability

that a flaw of a specific through-wall

penetration will be detected, and the ac- curacy of sizing through-wall penetration

once a flaw is detected. These are the

basic results available from existing eddy

current inspection systems. Inferences

from the inspectors, regarding the nature

of the flaws, their length, and/or other

U-Bend Test Specimen

Straight Section Without

Defect

Specimen

With Dent &

Flaws in Tube Support

Mockup

Straight Section With Notch

Defects

Specimen

With Dent &

Flaws in Tube Support

Specimen

With IG Cracks

Straight Section With Notch

Defects

Calibration Standard

Figure 17. Typical test configuration.

aspects of flaw characteristics are not

considered in this analysis. Also, the

ratio of incorrect defect calls versus correct defect calls are not considered at

this time. The probability of detecting a flaw, shown as the ordinate of the graph

in figure 18 is simply the ratio of the

number of defects reported divided by the

total number of defects present in the

specimen, i.e., probability of detection

Pd, at specific defect depth is:

flaws reported ^ flaws scanned

For the analysis of data in this

study, correct defect detection required

72
the flaw to be reported in proper sequence

and approximate location in the tube. In figure 18, probability of detection is

plotted as a function of percent of

through-wall flaw penetration, computed as the maximum depth of the flaw into the

tubing wall divided by the total wall

thickness.

Although the number of test samples

per data point was in some cases relatively small, particularly when the defects are categorized by particular geometries (pits, notches, etc.), each system

scanned similar defects approximately the

same number of times. Resultant trends in

relative system performance were therefore

considered val id.

This points to one of the difficulties in establishing system performance

curves for any one type of flaw geometry.

If three data points are taken at each 10

percent defect depth, 30 data points are

needed. When additional flaw geometries

are added, or when more data points are desired, the resultant number of tests and

required data analysis increases rapidly

to the point where a one-week test program, as conducted for each system in this

study, becomes impractical. Future

studies of this nature must therefore con- sider improved methods of rapidly scanning

and analyzing a large number of specimens

in a relatively short period of time, or concentrate on a limited number of defect

types.

The above analysis has one obvious

drawback. By presenting detection probability as a function of percent of

through-wall penetration, the influence of

flaw volume upon inspection results is not

readily apparent. In this case, a very narrow axial 60 percent through-wall flaw

could produce the same detection probability as a 30 percent deep wastage type

flaw covering a large volume of the tubing

wall. Since both of these defects can

have a different effect on tube integrity,

the practice of reporting defect detection

or sizing accuracy as a function of flaw

depth alone could be misleading in judging

the real performance of an NDE system for some applications. Although this is the

major analysis approach followed at the

present time, more comprehensive methods

of judging inspection system performance

are being considered and may be used in

future analysis of data.

100

80

.2 60

40

20

ABC-1 ABC-4

- —o—o——o—

BC-2 / ABC-3

A-3 -a B-4 C-3

WASTAGE

SYSTEMS

A c B& C <- SF

MF-2f, 3f

A-2

20 40 60 80

Defect Depth, (percent of wall)

100

Figure 18. Systems A, B, and C detection

of wastage.

In addition to detection probability,

the flaw sizing capability of an NDE test

needs to be determined. One approach is to show the mean error in percent of tube

wall thickness with plus and minus two

standard deviations, versus defect depth.

This approach is being used to analyze the

data in this program and the final results

will be presented in future papers and in

a special EPRI report.

Preliminary test results from evaluating the three systems are now considered in the next section.

5. Preliminary Results and Discussion

Analysis of the considerable test

data generated in this study is incomplete

at this time; however, the following preliminary results do indicate several interesting trends regarding defect

detection under a variety of test

conditions.

5. 1 Wastage

Figure 18 indicates the detection

performance of the three systems when used

to inspect steam generator tubing for

wastage- type defects. These flaws are in

straight sections of the tubing and not in

tube support or tube sheet areas. The

systems referred to in figure 18 and all

subsequent figures are:

System A - Zetec Inc. , single frequency (SF), conventional push-pull

drive unit, differential coil probe

(set at code sensitivity for these

tests)

73
System B - Holosonics/Intercontrole,

multiple frequency (MF), conventional

push-pull drive unit, differential

coil probe

System C - Battelle Northwest Laboratories, multiple frequency (MF),

conventional push-pull drive unit,

differential coil probe.

On figures 18 and 19, the number after the system designation (A-3) indicates

the number of independent tests conducted

on a flaw of a specific defect depth. The

data point presented in the figure is the

average of the set.

Since the wastage defects present

rather large volume flaws, the detection

probability was expected to be good for

all three systems, and it was. Detection

performance does not drop until the defect

depth drops below 20 percent of throughwall penetration for the single frequency

system.

100

s so

2 60

40

20

ABC-5 ABC-5 —

o

Single F versus Multi-F AXIAL NOTCHES

SYSTEMS

o 225 kHz. 750 LC Probe

ABC-5 |

3f (100, 240, & 400 kHz), Standard Probe

2f (200 & 400 kHz), Standard Probe

0 20 40 60 80

Defect Depth, (percent of wall)

Figure 19. Maximum single frequency test

sensitivity vs. standard multifrequency

sensitivity for detection of axial

notches.

Since the multifrequency3 systems

produce roughly twice the amount of inspection information and at two different

test frequencies per flaw per inspection

scan than does the single frequency (at

400 kHz), the detection probability is

expected to be improved. As shown in the

figure, the multifrequency systems do indicate a slightly improved detection capability for very small wastage flaws. It

is difficult to ascertain system performance

differences since the single frequency

results were also good. It is interesting

to note that the multifrequency systems

did not miss one wastage flaw in all the

tests. Detection of wastage- type flaws

represents one of the optimum uses of eddy

current testing and its performance in

this capacity has been very successful over the last several years in both com- mercial and military applications. The

results of figure 18 are therefore not unexpected and tend to confirm the validity of the round robin test program.

5.2 Single Frequency Parameter Study

The sensitivity used by the single

frequency system throughout the initial

phase of this test program was established

using ASME Section XI guidelines and is

representative of existing ISI test sensitivity. It is important to remember that

the single frequency results, presented

previously, were not conducted at maximum

equipment sensitivities. To investigate

the full capability of the single frequency system, a special series of tests were conducted in which the effects of

instrument sensitivity, inspection frequency and probe fill factor were considered. The probe fill factor is defined as the ratio of the diameter of the ET probe

squared to the inside diameter of the tube

squared.

A series of axial notches, 20, 40,

and 60 percent deep, were scanned by the

System A team members using various test

parameters. These narrow axial notches

(.005 in. width) are used to represent the

crack- like flaws parallel to the tube

axis. Although these flaws are perpendicular to the flow of eddy current in the

tube and, therefore, in a favorable orientation for detection, they are very low

volume defects and produce less response

than a wastage- type flaw of equal depth.

Since these defects are difficult to detect, they are ideal for system sen- sitivity studies.

After the above study, System A was retested against unknown defects using 225

kHz with a special probe having an outside

diameter of 0.750 inches. For this series

of tests, axial notches of 20, 40, and 60

percent were used. The test results are shown in figure 19.

3 Multi-frequency in this sense refers to a simultaneous coil excitation as distinguished

from sequential tests of a single frequency system at more than one frequency.

74
The two multi- frequency systems,

i.e., Systems B and C, scanned the same series of notches used to establish the

results in figure 19 and the subsequent

probability of detection curves are also

shown in figure 19. As shown, the

detection probability for all three

systems are identical. From this it

appears that the single frequency system,

if operated using the appropriate test

conditions, can approach the detection

capability of the multi frequency systems

for axial notches in straight sections of

tubing remote from tube supports.

Since past information indicates that

flaw sizing accuracy falls with decreased

frequency, simply dropping the test frequency of a single frequency test to in- crease flaw detection probability is not

always the solution to improved overall

inspection performance [5]. The type of

flaw expected, sizing accuracy requirements, and field experience, must all be

considered before the frequency and other

test parameters are selected.

6. Future Effort

Analysis of the considerable data

generated in this study will continue.

The first published report of the detailed

results will be presented at the Second

International Conference on Nondestructive

Evaluation in the Nuclear Industry,

February 13-15, 1978 (Session III, Problems Areas in NDE - Steam Generator,

February 13, 1978).

As a follow-on to this study, EPRI

has initiated the project RP1172, "Evaluation, Quantification, and Qualification

of Steam Generator NDE Technology." This

project will continue the SG NDE perfor- mance studies using an NDE evaluation

panel and air transportable mockup. This

12-month project is expected to begin in

December 1977.

This study would not have been possible without the excellent support of the

following members of the NDE evaluation

panel: A. Wehrmeister and H. Whaley,

Babcock and Wilcox; S. Brown, Battelle

Columbus Laboratories; J. Lareau, Southwest Research Institute; H. Houserman and

A. Sagar, Westinghouse.

These panel members provided extensive support in the areas of program planning, test conduction, reduction and interpretation of data, and analysis of system

performance.

References

[1] Van Rooyen, D. , "PWR Steam Generator

Tubing: Corrosion Problems," International Symposium on Application of

Reliability Technology to Nuclear

Power Plants, IAEA- SM-21 8/26, Vienna,

Austria, October 10-14, 1977.

[2] Steam generators to be replaced at

Surry, Turkey Point Nuclear Units,

Nucleonics Week, ]8, [22] (June 2,

1977).

[3] U.S. Atomic Energy Commission Regulatory Guide 1.83, "Inservice In- spection of Pressurized Water Reactor

Steam Generator Tubes," June 1974.

[4] Libby, H. L. , An improved eddy cur- rent tubing test, Report No. HW-81780, GE/AEC contract No.-AT(45

-1)-1350, Hanford Atomic Products

Operation, Richland, Washington (May

7, 1964).

[5] Sandona, E. and Denton, C. J., Eddycurrent examination of Beznau steam

generator tubing, in Nuclear Energy

Maturity . (Pergamon Press, Oxford

and New York, 1975) pp. 361-376.

[6] EPRI project RP403, Multi frequency

eddy current system for steam generator tubing inspection, Research and

Development Projects, EPRI Report, 67

(May 5, 1977).

[7] Beiers, T. S. , Ivey, J. S. , and

Romey, W. M. , "Phase I Report - In- vestigation of Parameters Influencing

the Accuracy of Eddy Current Test

Predictions for Nuclear Steam Generator Tubing O.D. Imperfections,"

MML-75-41 , Report for Empire State

Electric Energy Res. Corp. (Project

No. , EP4-2), July 18, 1975.

[8] Becker, R. and Holler, P., "Results

of the Application of the Eddy Current Testing with a Multi frequency

Method," Paper No. lb, Eighth World

Conference on Nondestructive Testing,

Cannes, France, Sept. 7, 1976.

[9] Buchanan, R. A. and Talbot, T. F. "Analysis of the Nondestructive Exam- ination of PVRC Plate-Weld Specimen

2512J." A Report for the PVRC Sub-
committee on Nondestructive Examination of Materials for Pressure Components, Pressure Vessel Research Committee, Welding Research Council, May

21, 1973.

[10] Lockheed-Georgia Company, "Technical

Manual, Nondestructive Inspection,"

AFLC program to Determine the Reliability of Nondestructive Inspection

Under Actual Field and Depot Conditions, F41608-73-D-2850-0038, May 26,

1975.

[11] Reinhart, E. R. , "A Study of In- Service Ultrasonic Inspection Prac- tice for BWR Piping Welds," EPRI Special Report, NO-436 SR, August 1977.

[12] Flora J. H. , and Brown, S. D. , "Evaluation of the Eddy-Current Method of

Inspection Steam Generator Tubing,"

Report No. BNL-NUREG-50512R, Battelle

Columbus Laboratory/Brookhaven Na- tional Laboratory Contract No. EY-76-C-02-0016, September 30, 1976.

[13] Weeks, J. R. , Materials performance

in operating pressurized water re- actor steam generators, Nuclear

Technology, 28, 348-355 (March 1976).

[14] Fletcher W. D. , and Malinowski, D.

D. , "Operating experience with

Westinghouse steam generators,"

Nuclear Technology, 28, 356-373,

(March 1976).

[15] Hare, M. G. , Steam generator tube

failures: world experience in watercooled nuclear power reactors in

1974, Nuclear Safety, 17, [2],

231-242 (March - April 1976).

[16] Weeks, J. R. , Corrosion of steam generator tubing in operating pressurized water reactors, in Corrosion

Problems in Energy Conversion and

Generation , C. S. Tedmon, Jr., ed. , (The Electrochemical Society, Princeton, New Jersey 1974), pp. 322-345.

[17] Birkle, A. J. , PWR steam generator

inservice inspections, in Third Conference on Periodic Inspection of

Pressurized Components . (I Mech E Conference Publications 1976-10,

London, England, September 20-22,

1976) pp. 61-65.

Discussion

Question (Mr. Endler): Is carbon steel

still being used in support plates?

Answer (Mr. Reinhardt): I think there are

j some vendors here. They might answer

that, regarding their new designs.

Comment (Mr. Houserman): Some of those

are being changed. There has been a lot of

study, not only on the material, but the

configurations.

Question (Mr. Mester): You mentioned

multi- frequency equipment did not do as well in some areas, or did better in

others. Was this the type of equipment

that Hugo Libby was describing?

Answer (Mr. Reinhardt): We tested the

Intercontrole pulsonic system from France. They came over and tested the mockup we

had. That was a commercial system, what I call a field test system.

We tested the system that is being

developed at Battelle Northwest which I consider, a development- type system. It

is more refined in technology than the

intercontrole system. It attempts to do

automatic analysis which is rather

signi f icant.

The Intercontrole system relies

heavily on a lot of manual analysis, but

it has been in the field several times.

So we had, a field-ready multi-frequency

prototype system and we also tested a laboratory prototype multi -frequency

system.

But our goals in testing the two systems were different. One, to see if the

field system could be taken into the

field, to solve some current problems. Our

role in the Battelle system was to help

direct their R&D in further development.

This is the first time that they had

really interfaced with the new problems,

and these problems are new. That is one of the reasons we conducted the round

robin for them. So there were different

goals in the two programs.

76
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977. Issued

January 1981

.

EDDY CURRENT TESTING IN GOVERNMENT

Patrick C. McEleney

U.S. Army Materials and Mechanics Research Center

Arsenal Street

Watertown, MA 02172

In preparing this presentation, the

first reference I reviewed was AMC P 702-

11, "Guide to Specifying NDT in Materiel

Life Cycle Applications." This handbook is

intended to serve as a guide to managers in

incorporating NDT in the management of

materiel at the different stages of the

life cycle. The document serves as a basic

reference to facilitate the planning,

selection, and application of NDT for ensurance of satisfactory performance at

reasonable cost. This document is cur- rently being revised and will be submitted

for DOD approval in the near future.

I turned to Chapter VI of the revised

document (draft) and under "Electromagnetic (Eddy Current) Testing" found only

ten ASTM documents referenced. Looking

further, under "General," I found two documents which pertain to eddy currents:

MIL-I-6870B Inspection Requirements, Nondestructive for Aircraft Materials and

Parts; and Air Force TO-00-25-224, Welding

High Pressure and Cryogenic Systems (Section 4). Subsequently, I found two other

documents which should have been listed

under Eddy Current Testing: MIL-T-15005E

for 70-30 and 90-10 Copper Nickel Alloy

Condenser and Heat Exchanger Tubing, July

1962; and T.0. IF-111A-36.

Proceeding, I reviewed the records of

the Defense Conference on Nondestructive

Testing, which originated in 1951 and will

in a few weeks hold its 26th conference.

I noted it was not until 1967 that they

recognized eddy current testing as a significant method and appointed an EC consultant to assist in the solution of

problems before the group. Prior to that,

it had been included in a battery of other

minor methods. There have been ten papers

on eddy currents presented at the various

sessions of the conference.

Several other indicators which I wi 1

1

not detail here demonstrate that the eddy

current method is not a major factor in the

design and procurement stages of the

government materials life cycle. Where the

eddy current method is most important is

during the service life of systems, i.e.,

in detecting defects that have arisen in

service. The standards used in most cases are the parts themselves with artificial or natural defects. They are used to help

define the condition of the parts and the

end of service life so that the system can

be withdrawn from service before failure.

The majority of papers presented at the Defense Conference dealt with maintainability (in-service) inspection. One,

however, (S. Friedman, NSRDC - 1974) dealt

with calibration standards and specifications for eddy current crack detection.

He noted an increased use of eddy current

instrumentation for' the detection and

characterization of cracks in structural

weldments. He also reported an apparent

dearth of adequate standards aimed explicitly at structural weld crack detection

effectiveness. Mr. Friedman concluded that

based on analysis of the experimental

results, it would appear that current

standards and practices in eddy current

inspection for cracks in structural weldments are generally adequate. However,

some easily implemented measures should be

taken in order to ensure greater effectiveness without any increase in false

alarm rate. These are:

1. Calibration blocks should

conform as closely as possible to the metal under test in terms of

electrical conductivity;

2. Instrument sensitivity should be

checked on a relatively wide

simulated crack in addition to checking it on a slit-saw cut or other, still tighter, simulated

crack;

77
3. Instrument sensitivity should be set with an insulating shim between crack and coil to simulate

maximum expected lift-off; and

4. Lift-off and instrument zero should be checked and reset, if

necessary, on sound base metal

structure prior to the inspection.

The shortcomings of calibration of eddy

current OD tubing inspection systems by

passing a tube containing fabricated flaws

through the inspection coil was noted in

MIL-T-15005E (Copper Nickel Alloy Tubing).

It is difficult to obtain accurate and

reproducible fabricated flaws in the

reference tubing.

In recognition of this problem, the

Naval Ships Engineering Center has sponsored a program at Battel le-Northwest to investigate alternate means of calibrating

eddy current inspection systems applicable

to 1 inch diameter, 0.070 inch wall coppernickel tubing. Two alternate calibration

approaches have been investigated:

1. the injection of reference

signals into the electromagnetic

field surrounding the eddy

current inspection probe by means of special coils and electronically developed signals

representing flaw conditions;

and

2. the production of reference

signals by translating specially

prepared metallic tabs and

electrically loaded coils past

the eddy current inspection

probe.

They concluded that artificially

generated signal patterns provide an alternative to fabricated flaws for producing eddy current calibration signals.

The signal injection and passive signal

generation techniques described for OD tubing inspection can provide the variety

of signal patterns necessary to confirm

proper operation and calibration of the

eddy current instrument under all anticipated inspection conditions. Signal

injection techniques can duplicate actual

flaw patterns, and passive loading coils or metallic tabs can closely simulate many

typical flaw patterns. Signal injection

coils can be incorporated into inspection

coil assemblies to permit periodic recall

of calibration data during or between

inspections.

Further, dynamic signal injection is

particularly versatile in that virtually

any signal pattern can be generated by

properly programming the semiconductor

memories. Once the memories are programmed, the information is stored indefinitely or until intentionally erased.

Complete libraries of program data or memory devices can be accumulated to ac- commodate particular test conditions or test criteria, such as tube material, size,

nominal wall thickness, and flaw types.

The memories are easily duplicated with

conventional PROM programmers at a small

cost in comparison to fabrication of

machined flaw standards.

In addition, metallic tabs and passive

loading coils are also attractive alternatives to machined flaws. Metal tabs are easier to fabricate and less costly than

machined flaws and can provide a good

variety of signal patterns for calibration

purposes. Loading coils and tabs can be

mounted on nonmetallic forms to permit ease

of handling and use. The signal pattern

amplitude and phase angle control that is

possible with loading coils using passive

electrical components presents some interesting possibilities for switching

arrangements to generate complete sets of

calibration signal patterns. Neither tabs

nor loading coils contain active circuitry

or require reference signals from the test

instrument which makes them more adaptable

to a variety of instrument designs. This

is in contrast to the signal injection

circuitry which must be tailored to a specific instrument design, although the

readjustments necessary to accommodate

most instrument designs are relatively

minor.

There are a few other points which I might briefly mention which are of considerable interest to the Government, although

not strictly of interest at this time.

Evolution of a defect characterization

scheme for eddy current inspection has been

impeded by the lack of an adequate model of

the magnetic field defect interaction

common to all the magnetic methods of

nondestructive testing. The major ac- complishment to date has been to show that

the magnetic field/defect interaction can

be modeled by finite element analysis

techniques including material nonlinearities and complex defect geometries. It remains to verify the results experimentally and to examine the feasibility of

78
applying the modeling technique to include

residual magnetism effects (for magnetic

particle testing) and alternating current

conditions (for eddy current testing).

One further point has come to my

attention in the last several months. Some

of my colleagues were preparing questions

for the DARCOM Eddy Current Level III Certification, and in reviewing the many

references, a plethora of terminology

became readily apparent. As one might

imagine, this creates much confusion.

Although much of this is what you run into

in physics texts, it was noted that some authors active in the eddy current field

have their own unique terminology. It is hoped that those taking the exam have all

read the appropriate reference material; if

not, they are definitely in trouble.

Discussion

Question (Mr. Berger): You mentioned the

biggest use of eddy current testing is

related to in-service inspection problems

in the field, and that the standards tend

to be actual parts which are defective.

Doesn't that make it very difficult to compare the calibration procedures of one maintenance depot with another?

Answer (Mr. McEleney): It doesn't help,

but the results have been satisfactory.

Question (Mr. Berger): Does one person

develop standards for the whole country?

Answer (Mr. McEleney): For example,

government personnel examined the whole

inventory of M-39, 20 mm gun tubes. There

were mixed lots of improperly heat treated

steels in the in-service pieces. These

barrels were distributed all over the

world, so they were examined in entirety,

using good and bad barrels as standards.

These standards were shipped around to various stations but all standards came out

of one place, Watertown Arsenal. There are

several other instances that are similar.

Usually, there is one source of standards

for a particular application.

79
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I National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977. Issued

January 1981.

CURRENT STATUS AND FUTURE DIRECTIONS OF

EDDY CURRENT INSTRUMENTATION

Tracy W. McFarlan

Ultrasonic & Electromagnetic Equipment

Magnaflux Corporation

Chicago, IL 60690

During the early 1950' s, eddy current

testing was introduced in this country to satisfy market requirements for a line of

electronic instruments that could be used

i for surface crack detection, sorting of

critical materials according to alloy

content, hardness, geometry variations,

I and direct conductivity determinations.

There was also a requirement for practical means of automatically testing parts

at production line speeds with automatic

readout of test results. Out of this

emerged several vacuum tube instruments

I with limited frequency ranges utilizing

; meter, cathode ray tube, and recorder

readouts, that were used for manual and

automatic testing of critical components.

These instruments were operated on 110 V lines, and they were considered portable

if they could be carried around by one or

two men. Operation required a skilled

man who understood the basic principals

of eddy current testing and electronics

and had the ability to interpret test

data.

ASTM established a subcommittee of E7 to develop a glossary and to write

recommended practice documents covering

critical industrial applications, and ASNT

provided good educational material in the

I first edition of the NDT handbook.

In the 1960's, solid state circuits

became available in the electronics industry and this caused considerable

changes in the methods of testing with

eddy current equipment. The instruments

were made much smaller and many of them

could be powered with batteries. Operation of the instruments was simplified,

automatic gates were developed, variable

frequency operation over much larger

ranges appeared, and readouts were improved considerably. Allied with these

developments were greatly improved me- chanical handling techniques that permitted reliable testing of materials of

many different sizes and shapes at high

speeds.

Today, the technology of eddy

current testing has improved to such an extent that we can test ferrous tubing

from the I. D. , automatically test steel

billets for longitudinal seams, detect

corrosion between first and second

members on aircraft structures, test

wire at mill speeds, test rods at elevated temperatures, automatically sort

large quantities of parts in automotive

plants, find microscopic cracks on complicated shaped aircraft structures

that cannot be found with other techniques, and accurately readout conductivity of materials in percent IACS.

Practical test specifications and

recommended practice documents have been

developed by several industrial companies, trade societies, and government

agencies that provide good test guidelines for many critical applications.

This work continues and, for the most

part, these documents are abreast of

events as they occur in the marketplace.

Personnel technical training facilities are available in many sections of our industry to educate people who are involved with eddy current testing from

the operator in the field to the supervisor who is ultimately responsible for

establishing set-up of the test and

action based on test results.

As for the future, all of us can

look into the crystal ball and see different things. It is apparent to us that the most important development must

be the instrument's ultimate ability to make more and more decisions on its own. The instrument will have the ability of

collecting sizeable quantities of test

data, and, properly programmed, it

should have the capability of digesting

the data and making an accept/reject

decision. Thus, pattern recognition,

accurate mechanical control of the probe/

coil with relationship to the test material,
storage of information and accurate calibration techniques will play an important

part. The computer and microprocessor

will have a considerable impact on future

instruments' designs because of their

control and decision-making capabilities.

We anticipate that the instrument will

have fewer controls, readouts will be much

easier to interpret, and the mechanical

portion of the system will become more

sophisticated. Also, operation of the

equipment at the extreme limits of the

frequency range will lead to solutions of

test problems that are unresolved today.

The future needs for improved eddy

current testing in industry are numerous. Time permits mention of only a few here.

Testing at elevated temperatures has

always been a problem because of the

difficult requirements for cooling the

probe or coil. A breakthrough in terms of

a new coolant or material used to make the

detecting element, that will withstand

high temperatures would be a big help.

Precision mechanical devices that can accurately move the probe or part through

the eddy current system to improve test

results are badly needed. Calibration of

the eddy current testing system in terms

of actual testing conditions must be

improved if we expect this method to become a more valuable and reliable test

tool

.

Calibration techniques that approach

the actual test conditions are highly

desirable. Of great importance is the

improvement of test specifications and

codes. Industry technical societies and

some government agencies have done a commendable job to date. This work must

continue with greater emphasis on the

practical application of the eddy current

system in the field.

Di scussion

Comment (Mr. Moyer): No criticism intended, but you have your rose-colored

glasses on when you say the suppliers will come up with our needs, especially when

our needs are very specialized. You will come up with the needs that will guarantee

Magnaflux or Magnetic Analysis or whoever

is the designer, the maximum dollar. If we could guarantee to buy enough equipment, you would come up with that need.

Answer (Mr. McFarlan): In our company,

after we have analyzed the marketplace

needs, all of the economic factors that

are involved, we make a return on investment calculation. If the ROI is satisfactory, we will design, produce and

market standard instruments. Now what

do we do with the instruments that are special? Or, what should we do with

instrument requirements where there is

not a big market; a production of five or ten instruments? To establish a product

for this market is difficult. Economically, we cannot afford to develop a standard product line. Instead, we design something on special order. But

the sales price will be higher than a standard product. This problem arises

because most companies who have special

requirements do not want to invest larger

amounts in special equipment. So you

negotiate back and forth. Sometimes

companies will build their own equipment

and other times they are willing to go ahead with the purchase of specialized

types of equipment from the supplier.

Question (Mr. Brown): Do you think the

microprocessor will make it possible to make fewer instruments that can be tailored in a wide variety of ways?

Answer (Mr. McFarlan): Absolutely, I think the microprocessors will find their

way into the "manually operated instruments" area that we have been working

with for crack detection. I can see microprocessors helping us to interpret

data. Bob McMaster put his finger on

it- -interpretation is killing us. The

microprocessor is going to be one of the

ways of obtaining better interpretation.

Comment (Mr. Brown): You should consider

the fact that you may be able to make one instrument for both eddy currents and

ultrasonics.

Answer (Mr. McFarlan): Right, we can design a combination system.

Question (Mr. Taylor): I was just won- dering whether you might make some predictions on what role the Bureau of

Standards might play in the future?

Answer (Mr. McFarlan): The NBS Conductivity Program is excellent. There has

been a dire need for the program for a long time. Beyond that program, if the

government gets too involved in the area

of NDT standards, it could represent a problem. I have talked to people about

this, and it is something we ought to bring out and talk about.

82
In industry we have a competitive

situation, and we like the idea of run- ning our own show. This is typically

American. There is fear in our minds

that the government will get too involved.

They are going to control the industry,

distort the standards, and try and tell

us how to do things.

The Bureau's contributions have been

very good. They have worked well within

i that organization. If that is an example

of how they are going to handle themselves in the future—great. We need

them.

Comment (Dr. Green): I would like to make a comment. I know from my own

j experience in working with the National

Bureau of Standards that everywhere they

I go they are viewed with awe and fear in

I the factories. People thought NBS was

going to regulate them. That is the

least of the Bureau's intent. I think

that is the least intent from the present

program. Of course, something develops

and someone else takes over and the

I intent can change. You cannot guarantee

that present policy will continue. I know at the present time, the Bureau does

not plan on being a regulatory agency.

Answer (Mr. McFarlan): Another area we ought to talk about is the bill before

Congress that nobody understands, that

might involve NDT.

Comment : I do not know what bill it is.

Question (Mr. McFarlan): If you people

know something about it and could en- lighten us, please do so.

Answer : My last information was that

bill was not coming out of committee.

Comment (Mr. McFarlan): I think we ought

| to know what it is and get some background on it. It is conceivable that if

it dies in committee now, it may show up

in the near future.

Comment : A comment on speciality systems. We are involved primarily in

special systems as a supplier, not only

! instrumentation, but material handling

| equipment. Oftentimes, we will develop

1 the proposal stages in very complicated

| highly engineered special systems, and we

will go to, in our case, steel companies

I with all of this work. They will take

the information, go out for bids on it,

and give it to the low bidder.

This is the name of the game, and we know it. There is really not much we do

about it. If Carpenter and Magnaflux

could enter into working relationships

for example, we are not spending a lot of

money for nothing; more could be accompl ished.

Comment : I think the bulk of us could do

with very simple instrumentation. Now

that we are developing complex dual

systems, I am all for them. But, let us put some software in them so that a high

school grad today can be trained to run

it. I am not saying that is always

necessary, but it should be true for the

majority of tests.

Comment : I agree with an earlier comment

about the willingness to invest large

amounts of money to get something that

will do the job. Unfortunately, too many

people rely on market analysis relative

to what the worth is of developing a new type of equipment for a new type of

application. At the start when one or two people have an idea to go some place,

the market does not look very big. I am sure Foerster did not know what his total

market was, other than the fact he knew

there were different applications. The

problem is we sometimes defeat ourselves

by market analysis. Once new equipment

becomes available, it is amazing how much

the market grows and becomes greater than

people first visualized.

Question (Mr. Weismantel): There is a great need in microprocessors; is Magnaflux pursuing this area?

Answer : Yes. In the computer area I can

tell you as of Monday of last week, we committed ourselves to an engineering

program to get into computerized NDT. Yes, we are in it. We have recognized

the possibilities for some time, but the

opportunity was not right, the timing was

not right until now. Now, we think it is right. Look at the trade shows and see what is happening, the ASNT show in

Detroit. It is quite obvious what is happening. Computerized systems were

there. That kind of acitivity spurs us on and motivates people to act.

Question (Mr. Berger): I find the

economics discussion interesting, cer- tainly a driving factor; but I would like

to get to some other aspects of the

instrumentation problem. Our problem is

measuring certain characteristics of the

instrumentation. We had a meeting on

83
I ultrasonics here a few weeks ago, as you

know, and one of the points made was that

there would be no attempt by anybody to standardize how everybody's pulser should

work, but there should be agreement on how to measure those pulses in terms of

rise time, shape, whatever. Are there

similar problems in regard to eddy cur- rent instrumentation?

Answer (Mr. Hentschel): First, one would

have to clean up the terminology. Everybody uses terminology that is diverse.

Question (Mr. Berger): You are saying

terminology is now in such a bad state

that we cannot agree?

Answer (Mr. Hentschel): Yes. The terminology has to be agreed upon.

Comment (Dr. McMaster): May I comment on that? Twenty years ago when we brought

out the first edition of the handbook, we

were voted down by two-thirds of every

group and they said the terminology was obscure, too theoretical, and too impractical to ever make it in the field. It

is better to make the word probe coil

standard. The handbook terminology

should not be frozen but, should in a paragraph say, probe coil, and then give

seven other different ways to describe

the same thing by inference. Then,

regardless of what literature a person

reads ten years in the future, there is a chance he will be able to recognize the

words. I detest frozen words of the type

you need to have in specifications.

Comment (Mr. Brown): I hate to disagree,

but there are some words like sensitivity, resolution, phase angle, that are things to be measured with numbers; and

they should have generally agreed-upon

definitions. We are way behind in this

field. And, when we get them, it will be

interesting to see what they describe.

Comment (Mr. Richardson): I would like

to comment. I believe the only thing you

can describe is the linearity of the re- sponse of the instrument. You can do

that with signal injection.

Comment : I am in the business of providing inspection services to the nuclear

industry. It would appear that NBS is

developing instruments that are going to measure conductivity, that is all right,

you should have some conductivity stan- dards to verify the performance of

tests. In the business of testing tubing

for flaws, there are problems. Standardizing the sorts of things we are talking

about is going to create more problems

for industry than it is going to solve.

Maybe because it is my business and I am particularly sensitive to it, but I am running up against ASME codes, the NRC,

etc. It is a fact of industry. We are meeting paperwork requirements. And even though we have a better test, a more

rapid test that gives more results, if it

does not meet these paperwork requirements, it is not accepted. The stan- dardization of these kinds of documentation hinders what we are all trying to do.

As an example, consider the ASME

code followed by the nuclear industry.

By the time something technical has been

developed and is accepted, published, and

accepted by NRC, there is a period of

about seven years. I do not want to see

us put in this position by NBS. In seven years trying to live up to standards we

set today, we have got to be very careful

of what we are documenting.

Question (Mr. Berger): I think you are

right. I do not have any problem with

that. I go back to what I said at the

beginning of the meeting. The kind of

standards or procedures we really like to develop, beyond the conductivity measurements, are those that contribute somehow

to better reliability or better measurements, or better assurance that what you

are measuring is there. It could be I various aspects of the instrumentation

are not important, and measurements of | the coil may not be important to a particular objective. But, are there mea- surements, calibration procedures, whatever, that can be made in the eddy

current field which would be helpful?

For example, the ultrasonic test blocks, j is there anything comparable in the eddy l current field?

i Answer (Mr. Denton): Right now, we have

what we call a master standard and we

have a mag tape on that standard, so we ! make ten more standards. Right now the

standards are traceable to my desk or your desk. If it is traceable to NBS,

j it is a lot more acceptable.

Comment : One problem we have is sorting | steel, grades of steel, due to slight

changes in chemistry, heat treats,

various things. If you could come up ! with some sort of standard or some mea- | surement, whether it is permeability, 1 saturation effects, whatever, to help us 1

84
sort grades of steel, bar stock, plate

stock, Bethlehem, J & L, etc. , which was made today or in the middle of the

winter.

Comment (Mr. Wehrmeister): To set a little parallel when you measure conductivity, as in sorting, you are looking

for a quantitive number to describe what

you are measuring. The standard that

developed through the years is a standard

of what it is you are measuring, regardless of what instrument you use, so you

can calibrate it to the standard. This

is where you have to draw the line. When

you get defect detection, cracks, things

of that sort to use parallel structure,

you would be standardizing cracks. And,

I have not found one yet. I think that

is where it ends. Standards end with

flaw detection, but it begins in sorting

appl i cations.

Comment (Mr. Brown): I know Mr. Denton

makes good standards. But there is a good probability he makes them the same way each time. What if somebody at the

other end of the country looks at the

boiler code and starts making standards?

How will they compare with the records in

your desk drawer? I have an uneasy

feeling about holes drilled at different

speeds, different temperatures, voltages,

etc. I think you could do a lot of good

by surveying the range of variation that

exists due to different techniques of

making holes. Do it over a wide variety

of conditions, fast, slow, EDM. Then the

areas in which standardization is needed

might be clearer.

Comment (Mr. Berger): We have done

something very similar with ultrasonic

reference blocks by borrowing all the

types of blocks we could find. The

variation was found to be about 40 percent. At least, we know what the number

is.

85
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5
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gai thersburg , MD, November 3-4, 1977. Issued

January 1981

.

MULTIFREQUENCY EDDY CURRENT INSPECTION TECHNIQUES

Hugo L. Libby

2302 Frankfort Street

Richland, WA 99352

1. Introduction

Eddy current nondestructive testing

methods are electromagnetic field methods

in which eddy currents are induced in the

test specimen by alternating currents

flowing in inspecting coils adjacent to or surrounding the test specimen. Test

specimen conditions are monitored by

measuring the impedances of the test coils

(or currents and voltages of the coils) as they are affected by eddy current flow

within the specimen. The methods have

been quite highly developed and they are

used widely in the metals industry for

the inspection of electrically conducting

materials and parts [I] 1.

Most eddy current inspections are made using single frequency excitation,

with the equipment permitting a selection

of any one of several different frequencies. However, it has been found that

use of two or more test frequencies simultaneously, resulting in a large number of

degrees of freedom in the signal, can give

a larger amount of information about the

test specimen than can be obtained using

a single frequency [2,3,4,5]. Other

workers are active in multi frequency eddy

current applications, but publications are difficult to find. Halmshaw [6] in an article on potential developments in non- destructive testing mentions work of R. Becker and P. Holler in Germany and refers

to various papers on the subject presented

in Session III of the 7th International

NDT Conference in Warsaw in 1973 and at the NDT Materials Conference at Nijmegen

in 1974.

A multi frequency eddy current tube

inspection system has been developed by

Intercontrole, Gif-sur-Yvette, France, but

the writer has found no technical articles

describing this work.

Significant work in the multi frequency eddy current techniques has been

done by Mr. Bob Meister and the staff at Battelle Columbus [7] using a different

approach than that reported in the body of

this paper. Their approach, a postanalysis procedure, uses an eddy current

system designed around a PDP 11/40 minicomputer and involves nonlinear transformation of measurements, application of the

transformed measurements to a decision

algorithm, and the display of results.

In contrast, the technique described

in the body of this paper is a real time

method. Much of this paper is based upon

work performed by Battelle Northwest [8]

sponsored by the Electric Power Research

Institute.

Important aspects of the eddy current

inspection method are design and construction of the eddy current inspection coils,

handling or transporting of the test specimens, selection of test frequency or frequencies, adjustment of instrument sensitivity, selection and use of test calibration specimens or standards, choice of

signal filtering means, test specimen

speed of translation, setting of any automatic alarm indicators, and interpretation

of test data when required.

Especially important in the design of

eddy current inspection systems is the

size, shape, and configuration of the inspection coils and the selection of inspection frequencies. Of the essence here

are the flow patterns of the eddy currents

which are affected by the factors men- tioned, as well as by the presence of

irregularities within the test specimens

which affect the electromagnetic properties of the specimens.

figures in brackets indicate literature references at the end of this paper.

87
The electromagnetic skin effect is

important in any eddy current system and

especially so in the multi frequency

method. It is the variation of the skin

effect with frequency and the resulting

differences in the flow pattern of eddy

currents that make it possible for the

multi frequency method to produce more information about the test specimen than

does the single frequency method [4].

2. Multi frequency Eddy Current Principles

Significant multi frequency eddy

current inspection principles are:

a. Two or more excitation frequencies are applied simultaneously to the

inspection coil assumbly.

b. The filtered, demodulated outputs

representing the response of the system to the different excitation carrier signals

can carry independent information as a result of the eddy current skin effect

which varies with frequency.

c. The principles of superposition

apply to the effects of the excitation

currents for inspection coils adjacent to nonmagnetic (nonferrous) test specimens.

d. The principles of small signal

analysis are usually applied in idealized

analyses because of the resulting simplifications.

e. The inspection of magnetic test

specimens is beyond the scope of the

present discussion because of extreme

nonl inear effects.

The principles of the multi frequency

(multiparameter) method of eddy current

inspection have been described [3,4] from

three viewpoints:

a. Generalization of the single frequency phase discrimination technique.

b. Algebraic solution of a set of

simultaneous equations.

c. Geometrical approach involving

vectors and signal space.

The interrelationships and common

basis of these three viewpoints will

become apparent as the discussion

proceeds.

2.1 Single frequency method

A single frequency eddy current

inspection device is depicted in figure

1. The single frequency generator A

Figure 1. Single frequency eddy current

tubing inspection device.

supplies excitation currents to the

differential internal eddy current probe

coils. The amplified bridge output

signal is applied to an amplitude-phase

detector which produces demodulated inphase (0°) and quadrature-phase (90°)

signal outputs C x and C 2 , respectively.

Two signals are shown in the output signal

plane C x versus C 2 caused as the inspection probe assembly is caused to trans- 1 verse past two hypothetical defects pi

and p 2 in sequence. Null bridge balance

j conditions are assumed except when the

probe coils are near the defects. It is j also assumed that small signal conditions

exist and that the Lissajous figures 1 formed are straight lines in the Cj versus

C 2 signal plane.

The following equations can be

written to describe the two signals C x and

C 2 when parameters p! and p 2 appear at different times:

auPi = Cx (1) i

1 a2iPi = c 2 (2)

a 12 p 2 (3)

a22 p 2 = c 2 (4)

where Cj and C 2 can vary as p x and p 2 fluctuate. The coefficients a lx , a 21 , a 12 , and a 22 are constants associated with the

inspection coil system, test specimen, and

general electronic signal circuits of the

instrument.

88
Equations (1) and (2) are written

assuming parameter p2 is zero. Similarly,

parameter Pi is zero for eqs. (3) and (4).

Now, assuming that the principles of

linear algebra apply, a consequence of

the small signal assumption, we can com- bine these equations into two equations

as follows:

a nPi + a i2P2 _ Ci

a 2lPl + a 22P2 = C 2.

(5)

(6)

This system of two simultaneous equations has two variables or parameters

Pi and p2

. The values of the coefficients

aUl . . . a2 2 can be determined by

varying p x and p 2 individually and measuring their effect upon the values of C x and C 2 , which can be observed. Equations

(5) and (6) can be expressed in matrix

form as

[A] P] = C] (7)

where

[A] =

a ll a 12

a21 a 22

P] =

Pi

P2

(8)

(9)

and

C] =

c 2

(10)

The matrix [A] is considered to be a modulation relating the test specimen

parameter P] to the demodulated output

signal C]. Excitation amplitude and

relative phase, amplifier gain and

instrument phase shifts are assumed to remain constant. The system of two equations, with known values of the co- efficients a., and the quantities c x and c 2 , can be^solved for variables p x and

p2 . A third signal caused by a third

variable cannot be accommodated in this

system of equations, as it could be

expressed as a linear combination of the

two existing signals and thus is not

independent. The output of the single

frequency system thusly can be analyzed to yield only two variables or parameters.

2.2 Multi frequency theory

It has been shown in a previous

paper [3] that the required independence

of signals may be obtained by adding

excitation frequencies and analyzing

circuits. Simple theory indicates that

for each additional frequency applied, two additional variables may be solved. How- ever, more advanced signal theory indicates that, assuming the test specimen

variables signal effects are of the mini- mum phase type, the number of additional

variables accommodated will approach one per additional frequency as the number of

frequencies are increased.

Assuming that we are considering a modest number of excitation frequencies,

the eq. (7) which was developed for a single frequency system can now be generalized to handle additional frequencies

by simply increasing the number of rows and columns in [A] and the number of rows

in P] and C].

The equation system in the form of

eqs. (5) and (6) may be solved using the

rules of algebra. The matrix equation

[A] P] = C] (7)

may be solved using the rules of matrix

algebra:

P] = [A]"1 C]

where [A] -1 is the inverse of [A].

(11)

The phase discrimination technique

is used widely in the single frequency

system to discriminate against a single

variable signal such as the one caused by

p 2 in figure 1 by rotating the signal

pattern by varying the reference phase

adjustment until the signal lies along the

C 1 axis. In this position, it has no component in the C 2 axis direction (line

Ci). Similarly, the parameter or variable

p x can be discriminated against by

rotating the pattern so that its signal

has no component in the C 2 direction. It

is noted that with the circuit shown the

two parameters cannot be separated at the

same time. This limitation is not one of

principle, and can be overcome by the use

of additional circuits. Also, this limitation is not inherent in eqs. (5), (6),

(7), and (11).

The foregoing concepts are exemplified and expanded in the diagram in figure

2. The inspection coil assembly is

89
PARAMETER SPACE P SIGNAL SPACE S

Figure 2. Multi frequency concept.

excited by a multiple frequency generator, and the receiver has several

amplitude-phase detectors whose outputs

are Cj. This is followed by a transformation section which performs the

inverse function indicated by eq. (11).

Figure 2 also applies the signal theory

concepts of parameter space, signal space,

and estimated parameter space to the eddy

current problem.

In the application of these concepts

in a practical instrument, the circuits

within the transformation section can be

adjusted manually to produce the required

parameter separating function. In the

foregoing discussion, the need to instrument the inverse of [A] is inferred. In fact, it is only necessarjy to instrument

adjoint [A] as it is the adjoint which

contains the variable separating capability or decoupling capability. This is stated algebraically as:

r/n -l _ Adjoint

" |A|

[A] =adj [A]

(12)

where I A I is the determinant of [A].

It is noted that |A| is simply an amplitude factor. In a practical example,

it is often desired to adjust the output

amplitudes at p x

. . . p individually.

2.3 Multi frequency eddy current device.

A multi frequency eddy current

inspection device is shown by the block

diagram in figure 3. It is shown to emphasize the nature of a two-frequency

system, but the extension in principle to a greater number of frequencies is shown by

the dotted lines. The diagram has been

drawn to emphasize that the signal outputs

—i \ ii

Figure 3. Multi frequency eddy current

tubing inspection device.

C x to C 4 of the receiver-detector

system can be considered to have come from two receivers, each operating at a different frequency. The output of the

first is Cx and C 2 , and the output of the

second is C 3 and C4 .

The transformation unit or section

can have any one of many different forms.

The one shown here in a line diagram is a direct instrumentation of the adjoint of

the matrix [A] previously discussed. The

line diagram indicates that each output

Pi is in general supplied by some com- bination of the inputs C-j. Provision must

be made to sum these input quantities in

various amounts of either sign including

the null quantity. The requirement here

for separation of variables is that the

summing circuits feeding any p. output

line be adjusted so that the effect of all

the remaining variables be discriminated

against at that particular output line.

Thus, in the example shown for four

variables, the summing circuits feeding

output line p x must be adjusted so that

the effect of variables p 2 , P3, and p 4 are eliminated at output line p 1 . Similarly,

the summing circuits feeding output line

p 2 must be adjusted so that the effects of

variables p 1} p 3 , and p4 are minimized at output line p 2 and in a likewise manner

for the remaining lines. Reaching the

desired adjustments is difficult as it is desirable to present to the inspection

coil assembly the several specific

variables, whose effects are to be minimized, in sequence for convenience in

each case.

90
It is emphasized that a fifth independent variable or parameter cannot be

separated using this four-parameter

; system. The system must be expanded by

adding another excitation frequency, a receiver channel, and additional transformer circuits to accommodate an additional parameter.

2.4 Transformation circuits

Two other forms of transformation

sections are shown in the next two

figures. Figure 4 shows one based upon

Cj 1

Figure 4. Parameter discrimination or transformation section.

the method of Gauss for eliminating one variable at a time [3]. This particular

circuit can discriminate against signals

caused by two variables (p x and p2 ) and

has two readout channels, one for p 3 and

one for p 4. This circuit, in contrast to the more general arrangement of the transformation circuit in figure 3, may re- quire interchanging of the input leads

depending upon the distribution of signal

components between the leads. This need

for increased flexibility arises because

the laws of linear algebra must be

satisfied. In operation the circuit is

adjusted (calibrated) to perform the

desired discrimination between signals.

The circuit in figure 4 is a four-parameter circuit and has the capability of

discriminating against two parameters and

of separating signals from two other

parameters at the two output channels 10

and 11. The signals caused by the first

parameter p 1 are minimized on lines 5, 6,

and 7 by adjusting in sequence the summing circuits at summing junctions a, b,

and c, while the signals caused by p x are being applied to the inputs at lines 1,

2, 3, and 4. Next, a second parameter

(perhaps some probe variable signal residue remaining on lines 5, 6, and 7) can

be minimized on lines 8 and 9 by adjusting

the summing circuits at d and e. The

inspection probe is now caused to traverse

past two selected defects, one representing outer wall defects, and one representing inner wall defects, and summing

circuits adjusted at f and g to give

optimum separation of the respective signals at 1 ines 10 and 1 1

.

The circuit shown in figure 5 uses Cartesian coordinate transformation

devices to perform the summing functions

[9]. The first parameter is discriminated

against following the same procedure as described in figure 4, except that rotation of the signal pattern rotator's

shafts (<)>!, <t> 2 , and <t> 3 ) are adjusted

instead of potentiometers. Rotators <J> 4 and (|> 5 are next adjusted to minimize

signals on lines 8 and 9 caused by two

other parameters. Next, 4> 6 is adjusted

to minimize signals caused by the fourth

parameter on line 11. Although the calibration (adjustment) of the rotators for

optimizing the discrimination between

variables is usually done experimentally

by the operator, the explanation of their

function can be clarified by following

signals through the circuit by an analytical method. This is done in the next

section. As an introduction to the

analytical approach, the equations describing the operation of the rotator

transformation unit are now given.

Figure 5. Parameter discrimination

section using plural transformation

rotars.

The transformation equations for the

transformation rotators shown in figure

5 are:

x' = x cos <|> - y sin <]) (13)

y' = x sin + y cos <J) (14)

where x and y are the two signal inputs

and x

1 and y' are the signal outputs, and

<t> is the mechanical angle of the rotator. Equations (13) and (14) are related to those used in analytic geometry to describe transformation of Cartesian coordi91
nates. However, eqs. (13) and (14) differ

slightly in that they have been modified

to describe the transformation of signals

flowing through the circuits rather than

the rotation of coordinates. Given a vector input signal with components x and

y, the shaft angle 0 which will produce a null (or near null) signal at the output

x

1 is obtained by equating the right hand

side of eq. (13) to zero and solving for

x cos <|> - y sin <|> = 0

SiS-J = * = tan + cos (J) y

Y

d) = tan ^ x

(15)

(16)

Equation (16) can be applied se- quentially to rotators, 1, 2, and 3 in

figure 5 to determine the values of

<(>!, <t> 2 , and <t>3 to minimize the effect of

the first variable on the signals dj,

d2 , and d3 on lines 5, 6, and 7. Next,

eqs. (13), (14), and (16) can be used to determine the values of <j> 4 and <t> 5 to discriminate against two more variables.

These two variables produce signals which

may occupy a two-dimensional subspace of

the main four-dimensional signal space

which applies in this example. When ty lt

(t>2, and <t> 3 are determined, the signals

dj, d2 , and d3 can be found for any input

signals to rotators ty lt <J> 2 , and <t> 3 .

The signal at e x is

e x = d x cos <$> 4 + d2 sin ty 4 (17)

and to minimize the signal at ej for a specific input signal S (in the two- dimensional subspace) which has components dlf d2 , and d 3 , we find from

eq. (16):

-1 (18)

d

l 4

= tan gl

(14):

The signal at line a is, from eq.

a = di sin <j> 4 + d2 cos <t> 4 (19)

The output signal at e 2 is

e 2 = (d x sin <J> 4 + d 2 cos <)> 4 )

cos 4> 5

- d 3 sin <J> 5

. (20)

We find the value of <t> 5 to give a null or near null output c 2 for selected

signal S in the two-dimensional subspace

by equating e 2 = 0 in eq. (20) and determine that

(dj sin <|> 4 + d2 cos <|) 4 )

sin <j> 5 COS <J> 5 = tan (|) 5

or

) 5 = tan

_^ d x sin <(> 4 + d 2 cos

(21)

• (22)

Finally, the output of lines 10 and

11 can be obtained by another application

of eqs. (13) and (14). The value of <t> 6 required to suppress the signals at the

output line 11 caused by signals occupying the two-dimensional subspace is

determined by application of eq. (16):

<|) 6 = tan 1

I

(23)

pq

When the rotator <|) 6 is set at the

value of $ given in eq. (23), the variable or parameter can be observed (when

present) on line 11 (signal f2 ). In addition, under the proper input signal

conditions, other signals not discriminated against will also give indications

at line 11. Results of the signals

caused by the three variables discriminated against will also be observed at

line 11. Amplitudes of these residues

depend upon the success of the discrimination adjustments.

3. Application of the Theory - Four-Parameter Example

The application of the foregoing

principles is illustrated in this section

by a combined approach using eddy current

instrument measurements of the responses

to fabricated tubing flaws and analyses

showing the projected performance of a transformation section with these

responses as inputs. The following

four-parameter example illustrates

the application of the multi frequency

principles described in the foregoing

section.

92
3.1 Data acquisition

This example uses signal data scaled

from Lissajous patterns obtained using a test specimen of Inconel 600, 7/8 in. diameter steam generator tubing containing

fabricated calibration holes. The wall

thickness T is approximately 0.050 in.

(1.27 mm). The calibration regions and

other conditions used in this report are:

0 one 100 percent T drilled hole,

0.067 in. (1.7 mm) diameter (T

equals wall thickness)

° one 80 percent T drilled hole,

0.078 in. (1.98 mm) diameter

° four 20 percent T drilled holes,

0.1875 in. (4.76 mm) diameter

spaced at 90 degree intervals

around the circumference of the

tube

0 a simulated tube support made of

mild steel 3/4 in. thick

surrounding the tube.

Lissajous figures were generated for

these tube inspection conditions using the

equipment arrangement shown in figure 6. (Signals observed of other calibration

holes in this tube section were present,

but results are not presented here in the

interest of brevity.) The equipment

comprised: (1) a two- frequency laboratory

eddy current system with a differentially

connected internal inspection coil probe

operating simultaneously at 100 kHz and

300 kHz; (2) a multiplexing electronic

switch; and (3) a cathode ray

oscilloscope. The in-phase (0°) and

quadrature (90°) demodulated (detected)

outputs of the two carrier frequencies,

100 kHz and 300 kHz, were applied to the

multiplexing switch. The purpose of the

multiplexer is to time-multiplex the four

output signal channels of the detectors so that two Lissajous patterns, one from each

of the two carrier frequencies, could be

displayed nearly simultaneously on the

cathode ray oscilloscope screen. Dwell

times of the electronic switches were

about 2.5 ms, and the repetition rate was about 10 ms, giving a display rate of

about 100 points per second for each

Lissajous pattern.

The measurement system used is equivalent to that of two single frequency

inspection devices, one operating at

MULTIFHEQUENCY GENERATOR

lOOKIIt 300KHi h/

Figure 6. Equipment for obtaining data

for algebraic solution of two-frequency

inspection method.

100 kHz having detector outputs C x and

C2 , and one operating at 300 kHz having

detector outputs C 3 and C 4 .

The patterns obtained by displaying

Ci versus C2, and C 3 versus C 4 by use of

the multiplexer were separated on the

oscilloscope screen by adjustment of dc

offset controls in the respective

detector circuits.

Tracings of several specific points

on the Lissajous pattern photographs for

several inspection conditions are shown

in figure 7. Signal components for these

conditions as scaled from the photographs

are given in table 1

.

200mv ^WOBBLE

4 20% T

, ,>-!>' 80% T-<.

a

1300KH 0

3 C3

*•

/ 4 100% T —V.''

3 V

>6

Figure 7. Signal loci for five test

conditions at 100 kHz and 300 kHz

obtained with multiplex system.

3.2 Objectives of calculations

The functioning of the transformation

unit will be shown by using the measured

instrument outputs as inputs to the

transformation unit and by calculating

the settings of the rotators and the

resulting signals in the transformation

section. More specifically, we desire to calculate:

93
Existing

Condition

Table 1. Components of Signals in Figure 7.

Si gnal

Relative Signals at Detector Output

Symbol C x (bj C 2 (b2 ) C 3 (b 3 ) C 4 (b 4 )

Probe $12 17. 9 48. 3 - 6. 0 229. 8 wobbl

e

Support $23 358. 7 -793. 3 -262. 6 -607. 6 $24 521 . 1 -576. 0 -112. 8 -715. 7

$2 5 513. 9 -179 7 50. 7 -564. 7

100% T $32 -227 6 -101 9 -380 6 -231

.

1 hole $33 -246 4 -166 0 -425 3 -449. 0

^>34 .10/1

1 04 U 1 CO U

QO 606. I

80% T §41 - 97 4 19 8 -116 0 100. 0 hole $42 -202 3 - 41 4 -367 5 30. 3 $43 - 87 2 - 33 0 - 98 9 - 13 5

4-20% T $61 -110 0 90 5 - 42 3 164. 8 4 holes $62 -232 0 79 1 -218 8 314 0 $63 -111 2 27 6 -121 4 155 3

a. The settings of rotators, ty lt

<j) 2 , and $3 to discriminate against probe

wobble signals.

b. The output of rotators ty l 3 <J> 2 , and <j> 3 for flaw signals caused by the

100 percent T, 80 percent T and the 4-20

percent T fabricated defects in the

standard tube, given the settings of

rotators <t> 2 , and <)> 3 for discriminating

against probe wobble.

c. The settings of rotators <t> 4 , 4>5, and <J> 6 to discriminate against the

support signals.

d. The components of peak signals

at the input of rotator <|> 6 (e x versus

e 2 ) caused by the defects 80 percent T and 4-20 percent T when (t> 4 and <J> 5 are adjusted to produce a null signal for the

signal caused by the 100 percent defect

(wobble discriminated against, but no discrimination against the support

signal s).

e. The relative amplitudes of the

peak output signals of the support (S 23 , S 24 , and S 25 ) and the 100 percent T, 80

percent T, and 4-20 percent T signals, all at output 1 ine 11.

Discrimination of signals will be

illustrated by use of the plural signal

rotator transformation unit shown in

figure 5. In operation, the rotators

01, <t> 2 , and <|> 3 are adjusted manually to

produce minimum probe wobble signal (while

the probe is caused to wobble) on lines 5,

6, and 7. These adjustments are made

individually, <)>! for line 5, <|) 2 for line

6, and <() 3 for line 7. It is found in the

example that the support signal is

effectively a two-dimensional signal,

occupying a two-dimensional subspace.

This signal is discriminated against by

adjusting iteratively the rotators <)> 4 and

<!>5 until the Lissajous pattern of this

signal when viewing the signals on line 9 versus those on line 10 (or lines 11 and

12) collapses to as nearly a straight line

signal as possible. Final discrimination

is obtained by rotating rotator <j) 6 to bring this nearly straight line to a horizontal position (minimum vertical

deflection) when viewing signals on line

9 versus those on line 10.

In this example, these adjustments

will all be calculated using the measured

instrument response given in table 1.

We do not have the same flexibility

in the calculations as we would have

operating the rotators manually because

of the large number of computations

required to simulate the iterative manual

adjustments.

3.3 Probe wobble discrimination

Equation (16) applies for discrimination against probe wobble:
tan"

1 S

y

(16) Probe Wobble S 12 where x and y are the input signals to a specific rotator and § is the rotator

angle setting to discriminate against the

signal vector x, y. Using the rotator

designations given in figure 5 and noting

from table 1 that the probe variable

signal vector is

S 12 = 17.9

we have

48.3 6.0 229.8

>! = tan

-1 jJ4 = 0.3706 fa = 20.34°

48. J (24)

fa = tan

-1

- 0.2102 fa =11-87°

229.8

(25)

fa = tan

-1 = 0.02611 fa = -1.50°. ^y.«

(26)

The outputs of the first three rotators,

the signals d 1( d 2 , and d3 on lines 5, 6,

and 7 in figure 5, are expressed by eq. (13) expanded here:

d 1 - b x cos <(>! - b 2 sin 4>i

d2 = b 2 cos 4> 2

- b 3 sin 02 (27)

d 3 = b 3 cos fa - b 4 sin (|) 3

.

Substituting the values of cos § and sin

<|> for 0X, <t> 2 , and (|) 3 in eqs. (27)

produces

d

] = 0.9377 b

] - 0.3475 b 2

d 0 = 0.9786 b„ - 0.2057 b„ (28)

0.9997 b. 0.0261 b4

.

Next, substituting the values of b 1} b 2 , b 3 , and b 4 of the peak signals S 12 , S 23 , S 24 , S 25 , S 33 , S 42 , and S 62 in eqs. (28)

gives the following values for d 1 , d 2 , and

d 3 , the signals on lines 5, 6, and 7 in

figure 5.

Support

100% T Hole

80% T Hole

>23

'24

'25

>33

'42

4-20% T Holes S 62

d2 d 3

di

d 2 d 3

di

d2 d 3

di

d2 d 3

di

d2 d 3

di

d 2 d 3

di

d2 d 3

= 0.00058

= -0.0035

= 0.0021

= 612.02

= -651.34

= -278.38

= 688.80

= -416.45

= -131.45

= 544.33

= - 59.70 = 35.94

= -173.36

= - 70.09

= -436.88

= -175.31

= - 46.75

= -366.58

= -234.61

= 12.82

= -210.53.

(29)

The signals d 1} d2 , and d3 for

vectors S23 , S 24 , S 25 , S 33 , S 42 , and S 62 are shown in figure 8 in three 2-space

projections, d x versus d2 , d 1 versus d 3 and d 2 versus d3 . It is noted that these

signals have been transformed from the

original values on lines 1, 2, and 3 to new values because of the discrimination

against the effect of probe wobble.

SnUBCE

Figure 8. Three 2-space views of signals in 3-space.

95
3.4 Null display of signal S42

e

] = 22.37

It is informative to determine the

display of e x versus e 2 (fig. 5), the

input to the final rotator <}> 6 when

rotators <J> 4 and <J> 5 have been adjusted to present a null view of the vector signal

S 33 , the 100 percent T signal. This view

will show the relative disposition of the

vectors S 33 , S 42 , and S 62 which are caused

by the three flaw conditions 100 percent

T, 80 percent T, and 4-20 percent T,

respectively. It will also serve to clarify further solutions used for

determinations of signals at e x versus

e 2.

The values of <^ 4 and <|> 5 to result in

the null view of S 33 at e 1 and e 2 are determined from eqs. (18) and (22) with

substitution of the appropriate component

values of the vector signal S 33 (d x = -173.36, d 2 = -70.09, and d3 = -436.88),

and the previously determined values of

sin <t> 4 and cos

-1 d

l d4 = tan gi

e 2 = -21.28

S 62e

l = 99,82

e

2 = -112.70

These and corresponding projections of

the d l5 d 2 , and d 3 axes are shown in

figure 9.

3.5 Discrimination against

support signals

It has been observed that the support signals can represent a two- dimensional subspace in the 3-space re- maining after discrimination against the

effects of probe wobble. This aspect of

the support signals can be illustrated

by taking the cross products of the

vectors S 23 , S 24 , and S25 (all at lines

5, 6, and 7) and then determine the an- gles between these cross products to find how closely the three vectors lie

in a plane in the 3-space.

(18)

d x sin <j>4 + d2 cos <j> 4

<t»s = tan 3 (22)

It is found that <J> 4 and <J> 5 may have

the following pairs of values:

<t>5 -23.17°

156.83°

-23.17°

203.17°

The last pair, Pair 4, gives the

proper aspect of the vectors S 33 (null),

S 42 , and S 62 when S 33 is chosen as the

null vector and the direction of view

is from the head of the vector S 33 to the origin. The signal values at e x and

e2 (fig. 5) for these conditions are:

S 33

e, = 0.00079

e

2 = -0.00396

<P4

Pair 1 67. 99°

Pair 2 67. 99°

Pair 3 247. 99°

Pair 4 247. 99°

Figure 9. Calculated display showing

peak signals.

Calculating:

u = S23 X s 24

V = S24 X S25

w = S23 X $25

96
and noting that

cos y

we find that

uv

cos yvw

U • V

U • W

IVI fwl

U • W

cos *uw " TUTTwr

Y = 2 85° J uv

Y = 1 47° J vw

Y = 1 88° J uw

Next, an average of three vectors U, V, and W is formed by adjusting their

component values to that corresponding to the average length of U, V, and W. The

resulting average is denoted vector D and

the relative value of its components are:

D = -3.1365, -12.6942, 23.2449. The

vector D is normal to the approximate

plane containing the three support signal

vectors S 2 3, S 24 , and S 25 on lines 5, 6,

and 7 in figure 5.

We will now find two vectors, both

normal to vector D and normal to each

other to be used to calculate the settings

of rotators <|> 4 and <j) 5 to produce a straight line (or near straight line) when

viewing the support signals by displaying

e x versus e 2.

Figure 10. Input to rotator <t> 6 with

null vector normal to S 25 .

A second choice was made for the null

vector at ex versus e 2 , based upon a small

vector model of the signals at lines 5, 6,

and 7. Inspection of this model indicated

that a more reasonable choice of null

vector would be one that is normal (or

nearly normal) to vector S 24 and, of course, occupying the approximate plane

containing the three support signal

vectors.

The vector D defines the subspace

(plane) containing the support signals in an average kind of manner. The vector S 24

is determined by solving the equation

S 24 • D = 0, (30)

where two components of S 24 are assumed to be equal to two corresponding components

of S 24 . We know from eq. (29) that

An attempt was made to build up the

first vector S 25 by assuming it to be near

S 25 . The rotator angles <t> 4 and (t> 5 were

then calculated to present a null view of

the vector S 25 x D at ej versus e 2 , as we

did for vector S 33 in figure 9. When this was carried through to completion, the

results shown in figure 10 were obtained.

(Calculations of this kind are discussed

in more detail in the next few

paragraphs.) The support vectors in

figure 10 have been transformed as desired

and they appear in a line, but the

projections of the 100 percent T, 80 percent T, and 4-20 percent T signals are separated by an angle of only 7° or less.

This is insufficient angle difference for

practical use. The resulting display of

these flaw signals also has a reverse

phase angle aspect, that is, flaws farther

from the metal surface produce signals

having increasing leading phase angles.

S 24 = 688.80, -416.45, -131.45 (31)

We then assume that

S 24 = 688.80, q, -131.45. (32)

Solving eq. (30) gives

q = -410.89. (33)

The vector S 24 is the first vector

needed which is normal to D. The second

vector normal to D, and the one to be used

as a null vector in the display which

discriminates against the support signals

is denoted S' and is equal to the vector

product:

So 4 x D = S'

97
It is found to be equal to

S

1 = 11,219.79, 15,598.86, 10,032.38 .

We find <J) 4 and <t> 5 by again applying eqs. (18) and (22) with d x = 11.21979, d2 =

15.59886, and d 3 = 10.03238 to give:

4> 4 = 35.73° or 215.73 °

4)5 =-62.43° or 117.57 °.

The underlined values are chosen as they

result in the desired aspect when viewing

the vectors in the e x versus e 2 display.

Again, the values of the other signals at e 1 and e 2 (fig. 5) given by eqs. (17) and (20) are:

S23 ei -877 17

e 2 167 44

s 24 ei -802 35

e 2 146. 20

S25 ei -476. 76

e 2 92 82

S33 ei 99. 80

e 2 314 09

s 42 ei 115. 02

e 2 260. 01

S62 ei 197. 95

e 2 128. 04

These results are shown in figure 11. The

results can be seen in better perspective

by now referring back to figure 9 wherein

the 100 percent T signal is viewed

"end-on," and the projections of the 80

percent T and 4-20 percent T signals are seen in accordance with their particular

orientations. Now, referring to figure

11, we can see that we are viewing the 100

percent T signal from some other

viewpoint, the one determined by the

vector S' , which presents us with one "edge-on" view of the support signal

plane. The support signals are thus

viewed as being very nearly in line. It

is now clear that the choice of vectors

within the support signal plane other than

S' as null vectors results in a rotation

of all signals around a line perpendicular

(normal) to the support plane and passing

through the origin. When this pattern is

rotated thusly, the projection aspects of

the signals in the e x versus e 2 plane

change greatly. The visualization of this

is aided by again referring to figure 9.

-800 ^00 ^So O S,

Figure 11. Input to rotator

vector normal to S 24 . with null

It remains, to obtain the

discrimination against the support

signals, to rotate the signal patterns in

figure 11 so that the support signals

produce a minimum signal in the ordinate

direction, or the signal h on line 11,

figure 5. This is done by adjusting

rotator 4> 6 . We note that the greatest

angle spread of the support signals is

between S 23 and S 25 , and it is 0.60°. The

angle of S 25 is

625 = tan"

1 = 168.98 .

Assuming we need to approximately equalize the maximum positive and negative

swings of the support signal, we should

rotate the pattern in the counterclockwise (positive) direction 0 degrees

where:

6 = 180 - 168.98° - 0.4325 = 10.588°.

The output of rotator 4> 6 is obtained

by using eqs. (13) and (14) which are rewritten here:

g =>! cos <|> 6

- e 2 sin <t> 6

h = e x sin 4> 6 + e 2 cos 4> 6.

When <|> 6 = 10.59° we find that

S23 g

h

$24 g

h

s 2 5 g

h

S33 g

h

-893.00

3.41

-815.56

- 3.71

-485.7

3.64

40.39

327.08

98
S42 g = 65.29

h = 276.72

S 62 g = 171.05

h = 162.23 .

These values of g and h are used to produce figure 12, and it is observed that

the support signals now have small com- ponents in the h (ordinate) direction, and

the remaining signals have a practical

orientation having the required phase

angle direction.

The h signal components are:

$33 h = 327 08

S42 h = 276. 72

$62 h = 162. 23

$23 h = 3. 41

1 00%T

S33

s„ s.

80%T

S42

4-20%T S„

SUPPORT

3 PROBE POSITIONS FABRICATED FLAWS ONE PROBE POSITION EACH

S 24 h

$25 h

3.71

3.64 .

-800 -600 -400 -200 200 400

Figure 13. Calculated relative values of

signals showing discrimination against

tube support signals.

Another way to explain this effect is to relate it to the theory of the multifrequency method. In this example, we

have a four-variable or four-parameter

system. Three variables have been

"eliminated," one for probe wobble and two

for the support. This leaves only one to be indicated at the output h. Of course, other variables not provided for by the

system will also produce deflections in

the ordinate direction.

Figure 12. Output to rotator <|> 6 with null

vector normal to S24.

These h components are plotted in figure

13 at a larger scale to show the large

amount of discrimination more clearly.

The amount of discrimination in this

example is good. Although the three flaw

signals in figure 12 appear to carry phase

information, it should be emphasized that

when the support signal occurs simultaneously with a flaw signal, the two are additive, and the horizontal (g component)

of the support signal is added to the g component of the flaw signal. Phase angle

information is retained when the flaw

signals appear remote from the location

of the support.

3.6 Simulated cross-section

display of tubing [3]

The multif requency method promises to make it possible to perform many

inspections not feasible with a single

frequency (one frequency at a time)

system. An example of this is shown in

figure 14 in which a four-parameter system

was used to discriminate against probe

wobble signals and to separate inner wall

and outer wall simulated flaws. Tubing

radial position signals obtained from a resolver produced a simulated pattern of

the tubing specimen on an X-Y cathode ray

oscilloscope provided with capability of z axis modulation. The output of the

four-parameter tubing inspection device

modulated the pattern to show the relative

location and relative severity of flaws

opening on the outer and inner walls.

99
metal saws or by

Grooves and other

made by machining,

producing notches

filing or machining,

irregularities can be

A much used method for

is the electric discharge machining process. Notches mea- suring a few thousandths of an inch wide

to simulate cracks can be made by this

method.

TEST SPECIMEN TYPE 304 STAINLESS

STEEL 2-'< IN O 0

'* IN. WALL

ARTIFICIAL FLAWS '.IN DIAMETER

HOLES IN OEPTHS SHOWN IN INCHES

Figure 14. Simulated display of tubing

cross section.

Drill holes do not represent exactly

the common types of defect found in

metals, but rather serve as convenient

alternates by which the sensitivity and

general performance of the inspection

device can be set or measured. It is

difficult to produce holes drilled

partially through the wall from the inside

of a tube. Notches are more costly to produce, but they can be made on the

inside of tubing and give a much better

simulation of cracks.

4. Calibration Methods and Standards

The same general principles of calibration and use of standard specimens

which apply to the single frequency eddy

current technique also apply to multifrequency techniques. However, with the more general method there are added com- plications resulting from the greater

number of degrees of freedom and the

associated larger number of specimen

variables processed.

The most highly developed standards

for eddy current application are conductivity specimens for calibrating electrical conductivity meters. In these

applications, two standards are sometimes

used for each meter scale, one for establishing a calibration point near the upper

end of the scale, and a second for a point

near the lower end of the scale.

Dimensional specimens are usually

made by the laboratory or manufacturer

interested in the application.

An area of increasing interest is

that of standards for tubing inspection.

Two conflicting needs are experienced:

(1) the desire to produce effective stan- dard irregularities which will cause

signals similar to those from specified

mill run flaws or to give other specified

response, and (2) ease of fabrication.

Drill holes are often used because of

the ease of fabrication and reproducibility of results. Holes are drilled

through the wall or partially through the

wall. Notches are sometimes made using

The nature of the calibration

problem is complex and certainly cannot

be treated adequately in this paper.

Continuing research is being done in

this area, and much more is needed. The

development of standards for the eddy

current tests have lagged behind that for

the other major nondestructive tests. This must be at least in part caused by

the abstract nature of electromagnetics

and the associated problems in interpreting the wide range of signal effects

observed. These, in turn, are a result

of the indirect nature of the eddy current

inspection. In many examples, the con- ditions which give rise to signal changes

are only indirectly related to material

or structural content.

The relationships between the calibration problems for the single frequency

method and those for the multi frequency

method can be clarified by referring to the section on principles. Let us first

examine the algebra describing the single

frequency technique. This is exemplified

by eqs. (5) and (6):

a n Pi + a 12 P2 = c i ( 5 )

a 21 Pi + a 22 P2 = c 2- (6)

These are simplified equations based

upon small signal (linear) theory. How- ever, they can be informative even though

they do not describe the behavior of

large eddy current signals. The co- efficients an, a 12 , a 2 i, and a 22 each may

depend upon all of the inspection

variables. For our present purpose, we divide these inspection variables into

100
two groups. Group A includes the factors

which can remain essentially constant

during an inspection period, and Group B includes the factors which usually change

during an inspection.

Inspection Variables

Group A - Essentially Constant

Probe assembly excitation

Instrument AC bridge adjustments

Instrument sensitivity

Instrument signal phase adjust

Instrument output channel sensitivity control

.

Group B - Usually Expected to Vary

Probe wobble effect

Effect of test specimen

Electrical conductivity

Magnetic effects (if any)

Specimen and coil

temperature changes (if

any)

Presence of flaws and other

irregularities within the

test specimen.

Because of our assumptions regarding

the fixed aspects of the coefficients a.,

in eqs. (6) and (7), we can consider them

to comprise a modulation matrix which

varies during an inspection mainly as a function of the inspection specimen.

During a calibration period, the elements

of this matrix are changed to new values,

then being functions of the calibration

controls such as gain or phase reference

settings. The single frequency inspection

output circuit has either one or two output channels. With two channels, the

main calibration effects are changed in the gain of either or both channels and a rotation of the pattern in the C x versus

C 2 display plane. Individual control of

the gain of the C x and C 2 channels cause distortion of the signal pattern.

In contrast to the single frequency

inspection technique the multi frequency

technique is more complicated in two main

areas. Firstly, the number of algebraic

equations (two for the single frequency

approach) is increased by two for each new frequency added. Secondly, the outputs of

the multiplicity of channels are further

processed through additional circuits

which are called transformation circuits

in this paper. These additional circuits

provide an involved mixing of the detector

outputs for producing the desired

separation of signals. The adjustment of

these circuits is done in the calibration

procedures. The equations applicable here

for the two-frequency system indicate the

increased complexity over that of the

single frequency example are obtained by

expansion of eq. (7).

a n Pi + a i2 ^2 + a i3 P3 + a i4 P4 = Ci

a 21 Pi + a22 P2 + a 23 P3 + a24 P4 = ^2

a31 Pi + a 32 P2 + a33 P3 + a 34 P4 = ^2

a 41 Pi + a 42 P2 + a43 P3 + a 44 P4 = C3 .

(34)

Again, as in eqs. (6) and (7), Cj...

C

n in eqs. (34) are the various detector

outputs. The next equation relates these

detector outputs to the final outputs.

t>n Ci + b 12 C 2 + b 13 C 3 + b 14 C 4 = Pi

b 21 C 1 + b 22 C 2 + b 23 C 3 + b 24 C 4 = P2

b 31 C x + b 32 C 2 + b 33 C 3 + b 34 C 4 = P 3

b 41 C x + b 42 C 2 + b 43 C 3 + b 44 C 4 = P 4

(35)

where the primed P

1 quantities indicate

the final outputs, the estimated values

of the parameters, and the b xl ...b 44 quantities represent the expansion of the

[A]-1 matrix in eq. (11).

Further insight into the effect of

the multi frequency system on calibration

can be seen graphically by referring to figures 10 and 11. The transition from

figure 10 to figure 11 is the result of

rotating the vector signal pattern in 3- space around an axis normal to the support signals and passing through the

origin. The effect of this rotation is to greatly change the relative angle separation between the three flaw vector

signals. In contrast, variations in the

101
reference phase adjustment in the single

frequency system cause just a simple

pattern rotations.

5. Future Possibilities

The future of the multi frequency eddy

current technique is very promising. It will produce results not possible with the

single frequency method. Multi frequency

equipment can be made to operate in

several different modes, including single

frequency modes. In its present stage of

development, multi frequency devices are more complicated and more difficult to adjust and operate than single (one at time) frequency instruments. The development of automatic calibration means is

needed.

The permission of Battelle-Northwest

and the Electric Power Research Institute

to use figures 4 through 13 and the basic

data given in table 1 is gratefully

acknowledged. Also acknowledged are the

helpful suggestions of G. J. Posakony,

Manager, Nondestructive Testing, BattelleNorthwest.

References

[1] Mc Master, R. C. , ed. , Nondestructive

Testing Handbook , Vol. II, Ronald

Press, New York (1959), Sections 36,

37, and 38.

[2] Libby, H. L. , U. S. Patent 3,229,198

January 1 1

, 1966.

[3] Libby, H. L. , Broadband Electromagnetic Testing Methods, Part IV,

Multiparameter Principles, BNWL-953,

Battel le Northwest, Richland, WA,

January 1969.

[4] Libby, H. L. , Multiparameter Eddy

Current Concepts, in Research Techni - ques in Nondestructive Testing , R. S. Sharpe, ed. , Academic Press, London

(1970) , pp. 345-382.

[5] Libby, H. L. , Introduction to Elec - tromagnetic Nondestructive Test

Methods , Wi ley-Interscience, New York

(1971) .

[6] Halmshaw, R. , Potential Developments

in NDT, The British Journal of Non- destructive Testing, 19, [1], 21 (January 1977)

[7] Multi frequency Eddy Current Inspection for Cracks Under Fasteners,

AFML-TR-76-209, Battelle Columbus

Laboratories, December 1976.

[8] Advanced Multi frequency Eddy Current

System for Steam Generator Tubing

Inspection, Report to Electric Power

Research Institute by Battelle,

Pacific Northwest Laboratories,

Richland, WA, 99352 (to be

publ ished).

[9] Libby, H. L. and Wandling, C. R. , Transformation (Analyzer) Device

Using Post Detector Signal Pattern

Rotators for Multiparameter Eddy

Current Tester, BNWL-1469, Battelle

Northwest, Richland, WA, September

1970.

Di scussion

Question (Dr. Birnbaum): I would like to get back to your comment about nonlinear

effects, even though your talk was not directed at that. Is there information

there that can be used, for example, to deliberately try to look at the nonlinear

effect as a flaw detection method?

Answer (Mr. Libby): Yes there is. I am limited here with the two frequencies, in

this example. But the more frequencies we would include, and using Taylor's approximation, you can say a curve is a sum total

of a lot of segments. So even though you

have nonlinear effects, if you have enough

information from the signal, then you can work with this. I think it is straightforward mathematically, All that I have

described here, I have done by hand — by

pocket calculator, but this can all be done

using algebra or through the computer. You

can use all of the regression techniques,

and nonlinear approximations. It is just

more difficult to do.

I have tried the method described with

magnetic materials with just a few frequencies and the results were discouraging.

But I think only because I did not have

enough information, enough different frequencies, to describe all of these curves.

Question (Dr. Birnbaum): I was thinking

more, for example, of using two frequencies

102
Ui and U 2 , and looking at the sum frequency

j which is created by a nonlinear interI . action.

Answer (Mr. Libby): This is done now. The 60-cycle testing of magnetic materials

over the years has made use of these

harmonics, and they are present there.

This is a relationship that can be

further explored. I feel this is just a start. We have just scratched the

surface. There are all kinds of possibilities and this is where the microprocessor will help to handle the

greater amount of information that can

be made available.

Question (Mr. Wehrmeister): Was this

system designed specifically for de-

' tection, or do you receive from it

information that provides defect analysis in terms of its depth?

Answer (Mr. Libby): So far, we use mainly the amplitude of the signal to determine the severity of the condition.

If I have several flaw conditions, more

than is accounted for by the number of

variables that I can handle, then any of

these flaw conditions appearing individually will show up with a phase

angle difference, like the 100, the 80

and the 20 percent flaws.

Now, that phase angle difference

will show up on the screen as long as I am translating the coil past those

flaws. But now, if I put the support

right on top of the flaw> the support

signal is in the horizontal direction.

Consider the flaw signals now. Even if they occur right at that same point as the support signals it will not change their vertical contribution at

all. But, in this case I have used up

all the degrees of freedom I am entitled

to. I have used one parameter for

wobble, two for the support, and I have

got one left to read out one additional

parameter.

But if I have three parameters, I say, there are the 100 percent, the 80

percent, and the 20 percent flaws. There

is some limitation here. I have to throw out the other two. Generally, you

do not have flaws appearing under the

support, so you can use the angle

information. But when they occur under

the support, then I can use only the

amplitude information.

Question (Mr. Blew): On your example,

there was a scaled amplitude of 200

millivolts. What is the actual amplitude

of the smallest flaw that you can handle

with a signal-to-noise ratio that would

allow you to detect it accurately?

Answer (Mr. Libby): Well, this varies a lot. It would vary with application.

It depends on the material and the

particular test, and the noise level.

Question (Mr. Blew): What would you think

the practical limitation would be?

Answer (Mr. Libby): Well, I do not know

what the limitation is, actually. We

have worked with flaws a few mils -- in

depth. But now if we are discriminating

against a support on the outer wall, the

tests are more insensitive to small

signals on the outer wall than in the

regular test.

Question (Mr. Blew): Would this have

been in about a mil?

Answer (Mr. Libby): I just cannot tell

you what the limitation would be.

Question (Mr. Blew): With practical

experience are you getting down into the

millivolt region of signal?

Answer (Mr. Libby): Well I think this

is just relative as' far as the millivolts. That depends on how much you are driving the coils, what the instrument

gain is. I had those units on the example

to help me in my calculations, so I car- ried through all those relative amplitudes

from the start, starting with the basic

data.

Question (Mr. Brown): From the standards

and calibration point of view, is it

true to say that you have to have a sample of each of the parameters that

you are juggling? If there are one or two things you want to get rid of, you

have to have a sample of them; and if

there are one or two things you want to measure out, you have to have a sample

of them. So, the standard for multiparameter testing might very well have

several different kinds of parameters on the standards.

Answer (Mr. Libby): And there is this

problem. If you are using a system like

the one I described where I am manually

adjusting, the electronics could be
arranged so that it will aid this

adjustment. That is something for the

future, where the processor, microprocessor would come in. You need to present these parameters in fairly rapid

sequence if you are doing it manually,

because you have several things to minimize. I have got to go across one,

two, three parameters, and if there is one

in there that I do not want to minimize,

then I have to remember that. And I must

minimize wobble at the same time. Of course you can do the wobble separately.

But, in the first generalized adjustment

where there are three knobs to adjust,

three or four parameters, then you have

got to wobble the probe at the same time.

You could do the wobbling, and then as you

pass the different parameter signals,

different flaws that you are calibrating

against, then minimize them that way. In some cases, you can do them one at a time,

but you have to be careful.

This is a more costly system, and it

is more complicated to adjust, more com- plicated to operate. But as it becomes

more automated, we can make automatic

cal ibrations.

Question (Mr. Berger): I think you really

answered the question I was going to

raise. Because your original answer to

his question implied that you needed a physical standard in order to calibrate

the distance. I was going to question

that. I think you could store in computer

memory what the signals would look like.

Answer (Mr. Libby): Like the system that

was described; yes, an approach like that

could be used. Yet, there are some subtle

things here. I do not want to over- simplify it in a few slides like this, but

it represents many years of effort. And

there were a lot of difficulties along the

way.

You must be careful. For example in

the wobble adjustment, I kind of glossed

over that. It was just stated that there

is wobble adjustment, which gets rid of

the wobble. And I said it just like that,

and it comes out beautifully on the slide.

But the output signals, especially the

phase angles between the final output

signals that I showed for those three

flaws, are fairly sensitive to the wobble

adjustment, because you are dealing with

all these different dimensions. Once you

adjust for it, then it can hold. But you

do not want to change your mind after you

have it calibrated.

Comment (Dr. Mc Master): If I may use the

board a moment, I want to mention one little thing I found that helped me on I wobble with probe coils and could be used i here to get that one nasty variable out. It applies to other coils as well.

To take a very simple case, our magnetizing coil might provide a certain

number of ampere turns or magnetizing

force or flux density or signal in the

vertical direction. And if we put in

ferromagnetic materials, we will increase

the flux, so that you get a larger resonance curve.

If, with respect to, say, a probe

coil and the surface, you have a liftoff,

S, and if you were to wiggle the probe up and down, say, through a modestly adequate

range to be greater than any effects of

surface displacement, it is possible to arrive at a very interesting situation.

If this represents the 100 percent signal

in air, in the absence of the test object,

it is something you can easily calibrate

an instrument to.

If this is your curve with the

ferromagnetic material present, then this

point also represents a vector of 100

percent magnitude. Notice, the phase has

changed. But you can wiggle from here to here with negligible change in the overall

signal. It sits there at 100 percent all

the time. So all you do is tune the

oscillator with the ferromagnetic object

in place such that when you wiggle this up and down there is no visible effect on your signals, and then read out a frequency of balance, if you will,

whatever you want to call it, a frequency

which restores 100 percent signal, which

often can be read out rather accurately.

I find very frequently when you read

out frequency instead of other parameters,

you get about five figures of stable

indication. So I have often thought that

in cases where wobble is a problem, if you

took it out at the probe by the selection

of the frequency which is automatically

self cancelling for liftoff of huge

amounts, and then went into what you are

doing, it seems it would be helpful.

Question (Mr. Bugden): If you use two

frequencies, is the relationship of the

two frequencies to each other, of great

consequence?

Answer (Mr. Libby): I like to work with

two to one or three to one, but we have

104
proved mathematically that as long as you

take different frequencies, no matter how

close together they are, the independence

of information exists. The signal-to-noise

ratio may or may not improve depending on the choice or frequencies. No matter how

close together you get these frequencies,

theoretically, there is some difference,

but when the skin effect becomes more equivalent for the different frequencies,

you have less difference in signals to work with.

Question (Mr. Bugden): Do you choose the

frequencies for convenience?

Answer (Mr. Libby): Yes.

Question (Mr. Mester): Have you determined the amount of liftoff that you can compensate for?

Answer (Mr. Libby): I do not have an exact figure, but it will compensate for a rather large amount.

Question (Mr. Mester): Has this work been

confined to ID coils or have you worked

with surface coils?

Answer (Mr. Libby): Well, I have used

surface coils for conductivity variation,

and variation in thickness, conductivity

being one variable, thickness the other.

We have used encircling coils, and small

probe coils. I said it was not good for

magnetic materials, but if, for example,

you have magnetic inclusions, just below

the surface of the material, it is very

good for detecting that; it eliminates the

wobble effects and some other variables

and detects the magnetic inclusions. Or,

if you wanted to cancel that out, and read

some flaw signals that occur near it, this

could be done. But this is when you have

a small amount of magnetic material.

Question (Mr. Mester): Is this system

described in a document that is available?

Answer (Mr. Libby): Yes, and there are references in it to some of the previous

work. There is also a reference to a new report that will be made available to the

public through EPRI, which includes this

work, plus additional information, but

that has not been issued yet.

105
Cur

Jai
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg , MD, November 3-4, 1977 Issued

January 1981.

PULSED EDDY CURRENT TECHNIQUES FOR

NONDESTRUCTIVE EVALUATION

D. L. Waidelich

University of Missouri

Columbia, MO 65201

1. Introduction

The pulsed eddy current system has

been used for the nondestructive evaluation of materials since the early 1950' s. Some of its advantages are much less

thermal drift and much greater resolution

than for systems using continuous sinusoidal waves. A typical system is described and some of the waveforms are presented. Also, it is shown that the

problem of lift-off may be overcome by

employing the idea of crossing points.

An equation involving the pulse length,

the material constants and the depth of

penetration is developed. Some results

obtained when testing non-metallic materials are given and also some recent

experiments with thick metallic slabs are discussed. Finally, various problems are presented and some suggestions are made.

2. Previous Work

Much of the early work on the pulsed

eddy current method was summarized in a recent reference [l] 1

. In the early

1950' s, eddy current systems used single

frequency sinusoidal sources, and the

subsequent heating of the probe coils led to thermal drifting which, in turn,

caused errors in locating defects. It was decided to use a pulse generator to drive the probe coil, and the thermal

problem disappeared immediately. There

was a new problem, however, in trying to interpret the results as viewed on the screen of a cathode-ray oscilloscope. It was found quickly that the defects near

the surface of the metal would show up in

the first part or head end of the pulse,

while those deeper in the metal would

affect the tail of the pulse.

A block diagram of a typical system

is shown in figure 1. The first systems

employed had a pulse generator driving a probe coil which launched the electromagnetic waves in the specimen of the

metal to be tested. The pickup coil re- sponded to the waves issuing from the

metal and containing the information con- cerning the defects in the metal and the

properties of the metal. The output of

the pickup coil was observed on an oscilloscope. Later, the output was filtered

in various ways, and an electronic gate

was used to pick out a particular portion

or portions of the output wave for re- cording. Recording may be done as a con- tinuous trace on paper or as a digital

readout printed periodically. The output

could also be entered into a computer for

further processing, such as digital filtering, the employment of a decision process, or an adaptive method involving

storage. Various forms of alarms or marking devices could also be actuated.

PULSE GENERATOR

DRIVEN PROBE

OSCILLOSCOPE

PICKUP COIL

RECORDER

METAL

SPECIMEN

FILTER AND

GATE

Figure 1. Pulsed eddy-current system.

figures in brackets indicate the literature references at the end of this paper.

107
The most usual circuit supplying

energy to the driven probe consists of a capacitor D which is charged slowly

through a resistor R from a dc power

supply as shown in figure 2. The capacitor is then discharged suddenly by a

0 c POWER

SUPPLY

R SCR

or THYRATRON ~]

C ORIVEN PROBE

COIL

1

Figure 2. Circuit supplying the driven

probe.

silicon-controlled rectifier (SCR) or a thyratron through the driven probe coil.

The pulse waveform is very nearly a half- wave sinusoidal loop as shown in figure

3, and the length of the pulse is determined primarily by the quantity -/EC where

Figure 3. The current through the driven

probe.

L is the inductance of the driven probe

coil and C is the capacitance of the

capacitor. The pulse shown is about two

microseconds long and has a peak value of

approximately 12 amperes. There is a little tail to the pulse which is caused

by the deionization of the discharge device. The shape of the current pulse is not very important because the metal acts

as a low pass filter. Consequently, the

higher harmonics contained in a rectangular pulse, for example, will be attenuated rapidly and the result in the pickup

coil will be nearly the same as if the

driving pulse were a half-wave sinusoidal

loop.

The pickup coil voltage is shown in

figure 4. Note that there is an initial

jump in voltage, then a sinusoidal shaped

Figure 4. The voltage across the pickup

coil

.

curve and finally an exponentially decaying voltage. Defects and changes in the

material properties change the shape of

the voltage from the pickup coil. These

changes carry the information about the

defect or the material property, and this

information may be abstracted by the

proper method.

One of the problems is the effect

of "lift-off." This may be overcome by

employing the idea [2] of the "crossing

point." If the pickup is placed directly

upon a metal specimen, part of the pickup

coil voltage of figure 4 is shown as the curve AA' of figure 5. As the coil is

Figure 5. Crossing point.

moved away or lifted from the metal surface, the trace of the pickup coil voltage successively moves from AA 1 to BB 1 , then to CC, and finally to DD 1 . The

crossing point 0, however, is not affected by lift-off, and so if the pickup coil

voltage is sampled electronically at the

crossing point 0, the output of the

sampler will be completely independent of

108
lift-off. The position of the point 0 will depend upon the presence of a defect

or upon the properties of the metal specimen, so its motion may be used to locate

a defect or to determine metal properties

without worrying about the effects of

lift-off.

3. Recent Work

One important relationship in pulse

work is that between the length in time

of the pulse, the constants of the materials being tested, and the depth of penetration of the electromagnetic waves into

the materials. In the Appendix, it is

demonstrated that

T = au D

2 (1)

where T = length in seconds of the pulse; a = electrical conductivity of the material in mhos per meter; u = magnetic permeability of the material in henries per

meter; and D = depth of penetration into

the material in meters. In some recent

work in aluminum and using pulses about a millisecond long, this equation has been

found to be useful in predicting the

depth of penetration.

Some work on testing poor electrical

conductors, such as plastics, indicated

that the above magnetic probes were not

very successful. The material seemed to have relatively little effect upon the

magnetic flux lines emanating from the

probes. It was thought that this type of

material might react more on the electric

flux lines, so capacitive probes were fashioned and simulated defects in plastic materials were detected by the use of

pulsed waves launched by the capacitive

probes [3]. Accidently, it was found that

these probes were also extremely sensitive in picking up and locating bits of

metal in the plastics.

Lately, experiments have been made

aimed at transmitting the waves through

an inch or more of aluminum and in detecting defects in a second metal layer

through about a quarter inch of aluminum.

Also defects in steel have been detected

through a quarter inch of metal by using

the set-up shown in figure 6. The magnetic field is generated in the steel by

using a coil wound on the elongated C- laminations. When the current in the

coil is cut off, the field collapses, and

the field is detected on the surface of

the steel by the small magnetic probe

coils. The presence of a defect in the

steel is indicated by aberrations that occur in the detected field. Some work

has also been done in detecting defects

in composites such as those made of

graphite.

DRIVEN COIL

METAL V-VN CRACK

PICKUP

Figure 6. System employed for ferrous

material

.

4. Questions and Suggestions

It appears as if further knowledge

is needed in the direction of what are

the limits as to the thickness of materials that may be traversed by the electromagnetic waves. Equation (1) may be modified by the state of the art, for example,

by the sensitivity of the detectors available and, undoubtedly, by the noise present. The question also arises, does

this equation or a similar one apply to poor conductors, .semi-conductors, and

insulating material? There is another

problem that occurs when defects are detected from one side of a material as compared to through-transmi ssions , and

this seems to indicate that the C of the

Appendix should be greatly reduced when

through-transmission is employed. There

is also a considerable amount of work

that should be done on the probes em- ployed. Two problems especially would be

important in this direction. The first

would be the development of better probes

for the poorer conductors and better insulators, and the second would be the investigation of the masks or shields for use with longer pulses to provide better

resolution. Further work is needed in the use of electronic gates, amplifiers,

and the devices that record the information. Also methods of decreasing the

noise are needed, and these would include

correlation methods and all types of filtering. A number of theoretical studies

would help greatly in understanding the

processes and in determining optimum

operation of the equipment.

109
References

[1] Waidelich, D. L. , Pulsed Eddy Cur- rents in Research Techniques in

Nondestructive Testing , R. S. Sharpe,

ecT (Academic Press, London, 1970),

pp. 383-416.

[2] Waidelich, D. L. and Huang, S. C. , The Use of Crossing Points in Pulsed

Eddy Current Testing, Materials

Evaluation, 30, 20-24 (January 1972).

[3] Decker, W. A. and Waidelich, D. L.

,

Nondestructive Testing Using an Electric Field Probe, in Proceedings

of the Seventh International Confer- ence on Nondestructive Testing , Warsaw, Poland, June 1-8, 1973,

Paper D-02.

Appendix

Assume that the surface of the material is the x-y plane and the positive z- axis extends into the material. The

conductivity of the material is a mhos

per meter and the magnetic permeability

is u henries per meter. The vector Helmholtz equation for the magnetic field

intensity H is assumed independent of x and y. In addition, the Laplace transform is employed to introduce the complex

variable s in place of the t in seconds.

Then

^-4 = ausH (A-l)

dz^

and the solution is

H = (Ho/s)e~

Z^CT|JS (A-2)

where Ho is the initial magnetic field

intensity at the surface (z = o) of the

material. Now the exponent of (A-2) must

be dimensionless , and since s has the

dimension of the reciprocal of the time T

T = Cap D

2 (A-3)

where the depth D is used in place of Z,

and C is a dimensionless constant. To determine the approximate value of C, known values of T, a, u, and D may be

introduced in (A-3). For example, it has

been found in nonmagnetic stainless steel

that pulses 2 microseconds long may be

employed to reach depths of 40 mils (1 millimeter) in the steel. Now if a =

1.1 x 10" 6 mhos per meter and p = 4n x 10" 7 henries per meter, then C =

1.447. The actual pulse length is not

at all critical, so for most work C is put equal to unity in (A-3). The actual

value of C depends somewhat on the state

of the art in that if more sensitive detectors are employed, D would increase

for a given T, and thus C would have to

be decreased. Also, the effect of the

noise present would change C. The above

value of C was obtained in detecting defects from one side of the material. If through-transmission is employed, it

appears from some tests as if the value

of C should be reduced to about 0.05.

Discussion

Question (Mr. Wehrmeister): In pulse

eddy current work, what are the effects

from acoustic energy generation in the

transmission of the pulse? Are some of

the time delays that you refer to the

acoustic energy being transferred to your

pickup coil, as opposed to the electromagnetic energy being transferred to the

pickup coil, especially in magnetic

material?

Answer (Mr. Waidelich): In all tests,

there was no actual contact with the ma- terial, so it would be rather difficult

to get very much acoustic ultrasonic type

transmission across the air between the

pickup coil and the material itself. Un- doubtedly, there is some motion in the

specimen, the metal specimen itself. But

I have seen relatively little effect that

can be noticed.

Question (Mr. Blew): In your work, when

there was a conducting film in the air or when there were films on a conducting

base metal, what would be the minimum

thickness that could be resolved, and

how close were the respective conductivities to each other?

Answer (Mr. Waidelich): The example, if

I remember, was zirconium on uranium. I do not remember the conductivities too well, but both are relatively poor conductors. The closer the conductivities

become, the more difficulty you have in

separating them.

110
Question (Mr. Blew): And what were the

relative thicknesses?

Answer (Mr. Waidelich): The thickness of

the cladding was about 30 mils, something

of that order.

Question (Mr. Blew): And the base?

Answer (Mr. Waidelich): The base was quite thick. I would say easily a quarter

inch.

Question (Mr. Mester): In the example

where you have a quarter inch of steel as the limitation of what you have penetrated, was DC saturation used during the

test?

Answer (Mr. Waidelich): No, we had not

used that in that particular example. We were going to do that in one of the

future experiments.

Question (Mr. Mester): What power levels

are involved with your inputs to your

driven coil?

Answer (Mr. Waidelich): We were using

approximately a 100 volt pulse. I cannot

tell you what the current was. We did

not measure the current.

Question (Dr. McMaster): How big were

the capacitors?

Answer (Mr. Waidelich): We used a number

of sizes. That is something like .01,

.05, . 1

, .5 uF.

Question (Mr. Bugden): Is there any particular ratio between the duration of the

pulse and the whole cycle that you find

beneficial?

Answer (Mr. Wadelich): It depends on how

thick a metal is tested. The electromagnetic waves penetrate and are reflected

from the back surface. The deeper a metal, the more time it takes for this

process to occur. This is indicated in

this equation to some extent. If you put

a particular pulse into a thick metal,

oftentimes the information you are looking

for comes a long distance after the input

pulse is stopped. You might find that you

get something like this in your pickup

coil --that is, a long tail. This tail can be quite long. The surface information is prior to this, and the deep information is way down here some place.

Question (Mr. Mester): You mentioned a thermal problem.

Answer (Mr. Wadelich): The heating that

exists when using sinusoidal currents

causes a lot of drift. The drift caused

difficulty in getting everything nulled

out. But, for one pulse the thermal

effect is relatively small. You can put a large current in this one pulse and get

quite a strong response. The only trouble

in doing this is the problem of trying to pick up the information afterwards. You

do not have the advantage of using all

the sinusoidal methods.

Ill

National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977. Issued

January 1981

.

THE INTRODUCTION OF SIGNAL PROCESSING TECHNIQUES

TO EDDY CURRENT INSPECTION

E. E. Weismantel

Quality Measurement Systems

The Aircraft Engine Group

General Electric Company

Cincinnati, OH 45215

1. Introduction

The use of eddy current techniques

for the nondestructive interrogation of

materials is not a new consideration

since the process has been in use to some degree for this purpose since the Second

World War. As with any advancing technology, the increased application of the

process results in a firmer definition of

its best uses as well as its limitations.

The basic process itself offers a very

high sensitivity for finding small material flaws in the near surface region.

However, because the process is very

sensitive, it also responds to other non- flaw type conditions that may exist

during the normal application of the

process. Typical major influences are: localized changes in conductivity due to alloy segregation, thermal effects,

or residual strain patterns within the

material, as well as factors that would

also affect the reactance of the system

such as coil to metal intimacy, part

configuration, etc. Such factors have

perhaps inhibited the broad application

of the process more than anything else

since, except for specific applications

where the effect of these other influcould be minimized or where the

sought were large enough as the effects of these other

eddy current process some- times developed suspicions as to its

reliability. The early uses of the

process were further hampered by the

characteristics of the early instrumentation that was available for its

application since the meter display of

this vintage integrated the effect of all

of these influences into a single meter

readout.

ences

flaws being

to override

factors, the

2. Current Trends

It has only been in the last five to ten years that advances in the process

technology itself,

availability of a electronic equipment

kedly broadened the

process.

coupled with the

new generation of

concepts, has mar- potential of the

This recently available equipment

allows the use of the impedance plane for

the analysis of eddy current signal

response. A typical impedance plane

presentation is illustrated in figure 1. This illustration shows the position of

various materials on the impedance plane

relative to their reactance and resistance effects on the' coil. Differences

in the electrical conductivities between

materials are illustrated on the resulting plots. The occurrence of flaws

within an alloy generally results in a small change along this curve. Coil -tomaterial spacing on the other hand, assumes a vector direction as shown

toward the "Air" termination point on the

conductivity curve. Thus, with the use

of the impedance plane, one gains the

ability to observe whether the coil's

reaction is due to a change in conductivity or due to coil to material spacing, probe wobble, etc.

A typical commercial instrument is shown in the next illustration, figure 2. This instrument has an added feature in

the rotational knob shown on the upper

left hand portion of its front panel.

This control provides a control over the

display which allows the rotation of the

impedance plane so that the lift-off

effect, for example, can be made to occur

in a specific coordinate direction.

Without the rotation feature, effective

use of the impedance plane relies on the

observational skills and attenti veness of

113
1000

900

800

700

600

500

100

200

.i£n MAGNETIC MATERIALS

.OlO""

\.006, '> .OlO"\.O03"

.006"

.003-

NON-MAGNETIC MATERIALS

LIFT-OFF FROM STEEL, TI 6-li, LEAD & 20U

•VECTOR NUMBERS INDICATE COIL TO METAL SPACING RFSISTANCE

300 100 500 600 700 800 900

Figure 1. Impedance Plane.

the output along the conductivity curve consists of both horizontal and vertical

movement. This is diagrammatical ly shown

in figure 3. If a two channel recorder

such as that shown in figure 4 is adapted

to the recording of output data from the

rotated impedance plane, the one channel

can be arranged to contain the vertical

movement along the conductivity curve while the second channel contains a com- posite of the movement in the lift-off

direction and the horizontal movement as- sociated with conductivity change effects.

A typical presentation of this is shown in

figure 5.

VERTICAL DEFLECTION

HORIZONTAL COMPONENT

OF CONDUCTIVITY

RESPONSE

CONDUCTIVITY ChANGE

OR FLAW RESPONSE CURVE

PURE LIFT-OFF RESPONSE

HORIZONTAL DEFLECTION

Figure 3. Representation of typical liftoff and conductivity change response on the rotated impedance plane - lift-off

horizontal

.

Figure 2. Typical commercially available

eddy current instrumentation.

the operator since both the lift-off

vector and the conductivity effects

contain both vertical and horizontal

components, and the plotting of horizontal

and vertical output data on normal

recording instrumentation would be of

little value for most applications.

However, the rotational feature of present

day instrumentation becomes of significant

importance to the process. With this

feature, the vector representing lift-off

conditions can be brought to react in an almost entirely horizontal direction while

Figure 4. Dual chart recorder used for

data gathering.

114
1

LIFT-OFF REACTION ESTABLISHED AS PURE HORIZONTAL MOVEMENT CHANNEL 2

VERTICAL COMPONENT OF CONDUCTIVITY CHANGE - CHANNEL 1

CHANNEL #1 CHANNEL #2

Figure 5. Illustration of typical recorded eddy current response.

3. Other Application Influences

Thus far, we have done little more

than to manipulate the impedance plane,

but from these illustrations the ease with

which we can derive more meaningful data

from the eddy current response should be

evident. Unfortunately, to this point, we

have been using idealized illustrations to show the significance of recent advances

in eddy current instrumentation and application methods with regard to the potential of the process. In facing real world

situations, even though these advances are certainly significant, a number of other

factors are encountered that further cloud

the interpretation of signal response re- sulting from the use of the process. Figure 6 shows a strip chart recording of the

typical eddy current response observed in

the inspection of turbine blade edges

using all of the innovations we have disI cussed to date. Certainly, even though a flaw signal is evident on this recording,

the chance for operator misinterpretation

remains high due to the confusing response

associated with the blade edge inspection.

The changes evident during this inspection

occur due to changes in electrical con- ductivity along the cast airfoil as well as changes in geometric configuration

from platform to tip. The eddy current

] process is sensitive to both of these effects as the recording shows.

0.070' LONG CRACK (NATURAL FLAW)

0.006" DIAMETER x 0.080" LONG HOLES - 0.003"

BEL0H THE INSPECTION SURFACE

FOUR EDM NOTCHES - 0.020" x 0.010"

Figure 6. Illustration of typical eddy

current response on turbine blade edges

without signal processing.

4. Overcoming Geometry and

Conductivity Effects

Sometime back when General Electric

first undertook the task of applying eddy

current inspection in the production en- vironment, we recognized that the variabilities we have discussed thus far might

affect the reliable use of the process,

and we, in fact, delayed the use of eddy

current inspection by production operators

until a more interpretable condition could

be established. With the cooperation of

the equipment vendor, we evolved into the

use of signal processing as a useful tool to further enhance implementation of the

process. Naturally, the first use of signal processing involved a black box which

is illustrated in figure 7. This addition

to the normal eddy current inspection

equipment already described yielded a dramatic increase in the interpretability of

the eddy current signal as is evidenced in figure 8. The upper portion of this illustration shows the signal response nor- mally resulting from an airfoil blade edge

containing known flaws. Although the re- sponse of the flaws is evident to a trained operator for most of the airfoil, cer- tainly as flaw size and thus the response

gets smaller, the chance for a miss on the

part of the operator increases. The lower

portion of this diagram shows the inspection of the same airfoil edge using the

115
processed signal. Clearly, the effects of

conductivity and geometry changes have

been all but eliminated, the flaw response

originates from a constant baseline, allowing a better judge of relative signal

amplitude, and the signals resulting from

flaws are clearly evident.

current inspection of a series of grooved

blocks —representative of a fillet con- dition. Three different alloys were used

in the experiment. Each groove contained

up to three fatigue cracks in a size range

varying from 0.010 to 0.250 inches. A total of 131 cracks was used in the evaluation. The result of this study is presented in figure 9. The superior flaw

detection capability of the process seems

to have been unaffected by the signal processing used for these tests. In fact, in

the small crack size end of the curve, the

detection efficiency of the process seems

to have been improved probably due to en- hanced operator interpretabil ity. With

this background, we introduced the use of

processed eddy current signals to the production inspection of blade edges more

than a year ago. Increased benefits of

further signal processing developments

appear to be obtainable.

Figure 7. Signal processor used with eddy

current instrumentation.

H0H-PR0CESSED SIGNAL

PMXESSED SIGNAL

Figure 8. Direct comparison of processed

vs. non-processed eddy current signal.

This very elementary approach to signal processing consisted only of the re- moving of the gradually changing responses

due to geometry and conductivity while

allowing the more discrete and abrupt

changes due to flaw conditions to remain.

Because we were concerned that these at- tempts to improve operator interpretabil ity might degrade the very excellent

flaw detection capability of the eddy cur- rent process, a statistically designed

experiment was undertaken to assess the

detection efficiency of the process both

with and without the use of signal processing. The test consisted of the eddy

KITH SIGN

WITHOUT SIGNAL PROCESSING

SIGNAL PROCESSING

INCREASING CRACK LENGTH ->~

Figure 9. Relative improvement in flaw

detection performance resulting from

signal processing.

Until now we have discussed things

that have already happened specifically

with regard to signal processing. These

improvements have resulted in the ability

to get some intelligence out of the eddy

current inspection data and have opened

the way for the further adapting of the

process to the use of the computer for the

further advancement of application technology. Typically, today's inspection of

cast airfoils uses a varying accept/reject

level depending upon where along the airfoil a response must be considered. The

most critical portion near the airfoil

fillet allows a maximum response amplitude

of 10 percent as shown in figure 10. Higher allowable amplitudes exist as one

goes outward from the platform due to the

lower stresses and lesser critical ity in

these areas. Currently, effort is underway on a semi -automated inspection system

which uses microprocessors to control the

116
movement of the inspection probes as well

as the acceptability of the signal re- sponse observed along the airfoil edges

relative to the probe's position at the

time the response is observed. Many other

possibilities exist for the application of

further signal-processing techniques to the process as we move to the future, but

one major unknown feature must yet be

recognized.

process. Even though current efforts to extend the technology should and will con- tinue, it is only through the enhancement

of our theoretical understanding of the

process that the value of multifrequency

testing and other advanced methods can really reach their full potential.

6. Summary

1. The introduction of the impedance

plane to practical use has opened

many avenues for improving the

interpretation of eddy current signal

response.

2. The rotation of the impedance plane

to differentiate the characteristics

of the response further enhances the

application of the process.

3. Signal processing methods can be

applied to improve the interpretability of the response.

4. With these accomplishments, the use

of eddy current inspection can be

tied to microprocessors and computer

control

.

5. Although marked advancements have

been made in the application of the

process much theoretical work remains

to be done to gain its full value and

potential.

Figure 10. Inspection criteria for typical turbine blade edge.

5. Looking Into the Future

Certainly the use and value of eddy

current technology has increased markedly

in the recent past. However, as we look

into the future we must recognize how much

more we have to know if we are to use signal processing of eddy current responses

to its fullest advantages. Currently, the

application of the process is guided by a few basic laws of physics and electronics

supported by an amount of empirical derivations and technical logic. Few of these

really define the conditions we experience

in the testing of the complex metallurgical

alloys which forms our every day work and

to which the process is to be applied.

Therefore, we have to work towards the

development of a theoretical understanding

of the process used in these situations

supported by computer modeling to help

predict the needs and performance of the

Discussion

Question (Mr. Judd): The signal processing device that was shown on one of

your slides, is this commercially available?

Answer (Mr. Weismantel): I expect it

probably is commercially available today.

It was developed on this program with the

cooperation of the equipment manufacturer.

He has a unique advantage in the fact that

it is applicable to his equipment but not

to some of his competitors' equipment

without going into the internals of the

competitors' equipment.

Question (Mr. Judd): Is this device es- sentially a wave shaper?

Answer (Mr. Weismantel): I did not fully

describe the total application of the

signal processor. It has several different functions. The one that I was describing involved the filtering out of
lower frequency occurrences that are associated with geometry and with chemistry changes. These occur rather gradually, whereas a flaw response is a very

sharp occurrence. And, that is about the

most simple approach that you can take to signal processing. The processor also removes electrical noise from the signal

as well as performing a number of other

functions.

Question (Mr. Lagin): Did you try any

pattern recognition schemes for detection

of dings, cracks, or grooves?

Answer (Mr. Weismantel): Yes, although we

are not applying these. Again, it is a situation where the people in the laboratory have developed some proficiency,

and are trying to transmit some of that

knowledge to people who might not be as flexible as the laboratory people. We are moving in that direction, and it seems to

be entirely possible, although what I say

has not really been totally proven.

Question (Mr. Lagin): The signal which

you actually process, is it like a signal

off a strip chart recorder?

Answer (Mr. Weismantel): No. It is the

signal sensed by the probe that is passed

through the flaw detector and then processed before it goes to both the os- cilloscope and the strip chart recorder.

Question (Mr. Lagin): But, it is quite

possible to use the signal processing

scheme on a signal in the recorder?

Answer (Mr. Weismantel): Yes. I do not think you have as big an advantage in

doing that, but yes, you could do that.

We also have a switch which allows the

signal processor to be switched in and out

of the system. This is especially

valuable if you are doing any analysis

work where the operator can still use the

oscilloscope to a large advantage.

The production system we have,

incidentally, inspects two blade edges at

a time. Both the leading and trailing

blade edges are tested simultaneously. It really has two parallel systems, two

signal processors, two flaw detectors, but

it only has one CRT since the CRT is

needed only for setup and for problem

analysis. I do not know what advantage

you would have in trying to process the

signal after it came out of the flaw

detector and oscilloscope but before going

into the strip chart.

Question (Mr. Lagin): The signals that

you showed, were typical signals that I would see on a strip chart recording.

You say they were on an oscilloscope?

Answer (Mr. Weismantel): Those were

strip chart recordings. But, that was because the signal had already been

processed and had been leveled out.

Comment (Mr. Lagin): As the strip chart

recording is progressing in time, you

obtain a very sharp signal over a small

amount of time on a defect. For a groove, it would be a slower type signal

and you could perform image processing

techniques on a signal like that.

Answer (Mr. Weismantel): Yes. I am not saying you cannot do that. The problem

you have is that the response from some

flaws can resemble the response of a groove or notch or ding, except that they

might move in one direction or the other

as far as their first movement is concerned. Not all responses start out in a positive direction, but the motion seems

to be related to the character of what

you are encountering.

Question (Mr. Houserman): The graphs you

presented on detectabi 1 ity, were the

statistics gathered from a production

type operation?

Answer (Mr. Weismantel): Yes.

Question (Mr. Houserman): With the

people that typically do the measurement?

Answer (Mr. Weismantel): Yes.

Question (Mr. Houserman): Secondly,

would the data show that with signal

processing you are getting more false

alarms?

Answer (Mr. Weismantel): No, actually we

are getting less. We delayed the introduction of the process to production

until we had the signal processor, be- cause during the time we were initially

looking at this problem, we recognized

the very high rate of false encounters

that we were having. At one time without

the signal processor, false signal alarms were encountered on approximately 10 percent of the parts processed. With the

signal processor it is about 1 percent,

and we think we have a better product.

Question (Mr. Denton): Looking at the

data on this strip chart, it appears your

118
signal processor is really a resistor and

capacitor differentiator. Is that true?

Answer (Mr. Weismantel): Not entirely.

Question (Mr. Brown): The curve that you

showed flattened off on the right side.

Did you increase the size of the crack?

If your crack was several inches longer,

a foot longer, does it drop off?

Answer (Mr. Weismantel): Not that I know

of. But, since we have not had any

cracks that are that long, I do not have

data to show that.

119

National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977. Issued

January 1 981

.

DEVELOPMENT OF NON-FERROUS CONDUCTIVITY STANDARDS AT BOEING

Art Jones

Boeing Aerospace Company

Seattle, WA 98124

1. General

The need to verify the accuracy of

eddy current meter readings which are used

to determine the physical characteristics

of non-ferrous alloys by measuring their

electrical conductivity is fully accepted

by industry, both manufacturers and users. The eddy current meters, therefore, must

be accuracy certified by means of NBS 1 traceable conductivity standards for the

readings to be both reliable and repeatable. Figure 1 shows a typical graph of

tensile strength vs. %IACS (conductivity)

for aluminum. To calibrate and accurately

AVERAGE VALUE

, 95% CONFIDENCE LIMITS

29.5 30.0 30.5 31.0 31.5 32.0 CONDUCTIVITY (% IACS)

Fi qure 1 general relation between tensile strength and eddy

' CURRENT CONDUCTIVITY MEASUREMENTS OF 2024-T4 ALUMINUM certify these eddy current meters, some- times called conductivity meters, the

Boeing Company embarked upon a research

and development program in 1966 to produce

their own NBS traceable non-ferrous con- ductivity standards, since none were com- mercially available at that time. Also,

NBS was engaged in providing only commercial copper conductivity standards at that

time so it was necessary for Boeing to obtain indirect traceability by means of

dimensional, resistance, and temperature

standards. All of these NBS traceable

standards were available at the Boeing Me- trology Laboratory (BML) in Seattle, WA.

Figure 2 shows the chain of traceability

to NBS from dimensional, resistance,

and temperature standards.

CONDUCTIVITY Sl*NOA»D! TBACEAgUITY TO NBS

Figure 2.

2. Definition of %IACS

The accepted definition of commer- cially pure copper as stated in NBS Copper

Wire Tables #31 is: a copper rod, one meter long having a uniform cross-section

of one sq. mm and a resistance of 1/58 ohm

at 20 °C is 100 percent International

Annealed Copper Standard, or 100 %IACS.

Using this as a reference then, all other

nonferrous metals can have their conductivities determined relative to the hypothetical 100 %IACS value. By using the

formula

P =

RA

L

where: p volume resistivity, in ohm

cm2 per cm,

x The National Bureau of Standards, U.S. Department of Commerce.

121
R = resistance in ohms at 20 °C

of a particular length of

uniformly dimensioned non- ferrous metal

,

A = the cross-sectional area in

square centimeters, and

L = the length being measured,

for the resistance R, in

centimeters

we can solve for volume resistivity. Converting the area and length to microhm

centimeters, and using the definition for

100 %IACS above, we get,

figure 3 for a typical indirect reading

type eddy current meter. Traceability to

%IACS 172.41 (a constant)

unk volume resistivity (in microohm centimeters)

This is the general equation for finding

the relative conductivity in %IACS for all

non-ferrous metals from their dimensions

and resistance at 20 °C.

3. Historical

The first use of an A-C probe coil

method to measure the electrical conductivity of non-ferrous metals was made in

1939 by German industry. Subsequent improvements in lift-off and sensitivity

increased both repeatability and accuracy

so that the eddy current meter finally

came into its own as an important non- destructive testing tool in the early

1 960 1 s . The sorting of known non-ferrous

alloys, mostly aluminum, and the verification of their proper heat treatment was

now possible with speed and with moderate

accuracy. Unknown alloys might have to be

verified by spectroscopic means because of

the overlap in conductivity values between heat-treated alloys of one composition and nonheat- treated alloys of a different composition. Once the alloy was properly identified, eddy current testing

could take over the task of determining

the correctness of heat treatment.

Boeing first used eddy current meters

for crack detection in the late 1950' s. However, it was not until heat-treat

identification of special alloy hydraulic

fittings, called "B nuts", was urgently

required in 1962 because of stress corrosion problems that we began to use eddy

current meters for heat-treat identification of aluminum alloys. One such meter

was a Magnaflux ED-500 which required the

use of curves for conductivity values. See

Figure 3. Indirect reading type eddy

current meter.

NBS or any other primary standard was not available at this time. Direct reading

eddy current meters were employed later on (except on "B nuts") when conductivity

standards covering the %IACS span of

interest, usually aluminum, were made

available. Calibration of these meters

with only "end of scale" conductivity

standards can result in large mid-scale

errors. The two standards usually provided on direct reading eddy current

meters claim neither accuracy nor traceability. See figure 4 for a direct reading type eddy current meter and a set of

- OB J,

Figure 4. Direct reading type eddy

current meter.

secondary conductivity standards. In late

1974, I received a letter from HawkerSi ddeley Aviation Ltd. of England indicating a 2 percent IACS difference between

U.S. and French Aerospatiale standards of

conductivity. On the national scene, I have witnessed differences of nearly 1 percent IACS between Boeing secondary

standards and those of other U.S. manu122
facturers. Both of these types of discrepancies are intolerable because of the possibility of allowing improperly heat

treated metals to be used in aircraft

structures with the possibility of dangerous or fatal consequences. Figure 5 shows three sets of Boeing non-ferrous

conductivity standards in carrying cases

for 2,3, or 8 standards.

current and potential clamps, was made

with an estimated accuracy of about 2 percent of reading at the normal lab tem- perature of 23 °C ± 1°. Dimensional area measurements and the spacing between the

inner potential clamps were made by the

Physical-Mechanical Section of BML A laser interferometer was used for mea- suring the distance between the two inner

clamp marks. The indeterminate position

of these marks is why the accuracy was relatively poor. Later, length measurements made were far more accurate and used

a laser to measure the distance between

two very thin scratch marks on a soft

aluminum bar. All later dimensional

measurements were made at 20 °C ± 0.5 °C. Figure 6 shows the method used for

thickness measurement, and figure 7 shows

the laser interferometer method for

determining the effective length.

Figure 5. Three sets of Boeing non- ferrous secondary conductivity

standards.

4. Initial Standards Requirements

The need to make rapid non-destructive tests on incoming non-ferrous

material has become increasingly important

because incorrectly heat-treated alloys

can fail in service and have even been

suspect in some collapsed aircraft nose

wheel accidents in the late 1950' s. In order to insure the accuracy necessary to properly categorize both the raw stock and

finished material, BML was assigned the

task of producing accurate non-ferrous

conductivity standards which had

traceability to NBS.

5. Preliminary Steps

The first involvement with eddy

current conductivity standards at Boeing

Metrology Labs came in 1966, when

personnel of the Boeing Airplane Division

(now the Boeing Commercial Airplane Company) brought some 1 in x 44 in x 1/8 in

aluminum bars of various alloys to the

primary standards laboratory to be measured. These bars were produced as a result of the requirements of the

MIL-A-22771B government specifications on aluminum forgings. A somewhat crude

measurement, using L&N List No. 4308

Figure 6. Thickness measurement.

Figure 7. Laser interferometer length

measurement.

The resistance measurement was made

using an L&N six dial double ratio set, an adjustable reversible, direct current

source, a sensitive null detector and any

123
one of three NBS traceable shunts: 0.01,

0.001, or 0.0001 ohm as required. Figure

8 shows these shunts in their oil bath. A separate, stirred, temperature-controlled

shunt oil bath, a double ratio set, and a specially designed primary standards

conductivity bar oil bath were used for

resistance measurement. The latter oil

bath was developed at a later date

(approximately 1967) as part of the

Figure 8. Precision shunts in stirred oil

bath with 6 dial double ratio set.

overall research and development

program to produce traceable primary

standards reference conductivity bars.

Figure 9 shows the conductivity bar

Figure 9. D-C primary bar calibration

facility oil bath.

DC calibration fixture on a rack above its

oil bath, ready for a primary bar to be

inserted in place. Figure 10 shows

current and potential connection details

with a primary bar in normal position.

6. Primary Standard Bars

To provide the required accuracy for

working standards of conductivity, it was first necessary to fabricate and certify

primary bars from the best possible

commercially available materials using

both known and newly developed techniques.

Copper, aluminum, bronze and titanium

sheet stock 0.25 in thick approximately

2.0 in wide and 60 in long was cut and

careful ly fabricated using as reference

Figure 10. Current and potential

connection details.

the ASTM Description B193 method for

volume resistivity. This is an absolute

method utilizing dimensions, resistance,

and temperature for determining volume

resistivity. Figure 11 shows 18 of the 19 primary standards conductivity bars in the

shallow transfer oil bath. The lid is

removed to show the bars in place. This

bath is used for 100 kHz calibration of

secondary standards which is described

later on in this paper.

Figure 11. Primary standards conductivity

bars in shallow oil bath.

124
7. Primary Conductivity Bar Stability

When the eight primary conductivity

bars were constructed in 1966, it was envisioned at that time that there could

be small but perhaps significant

alterations in the initial values due to dimensional variations caused by solid

state changes. Additionally, the

resistivity of some of the alloys could

also vary due to microstructural changes.

Errors such as grain direction and

stratification, which was discussed in ISA

paper 70-613, "Error Analysis of

Non-Ferrous Conductivity Standards," could

change in value with time and also cause

some change in the certified bar values.

These latter two errors were not reverified since the original data was

taken, but care was taken in the selection

and fabrication of the newest bars to minimize these effects. In order to determine what parameters had changed in

the bars themselves and to decide how much

effect these changes had on the certified

conductivity values, it was necessary to measure all of the bars again both

dimensional ly and electrically under the

same tightly controlled conditions as in

the original calibration.

The following table 1 shows the

changes in the dimensional and electrical

values of the original eight primary bars

and the change in the individual certified

values over an eight year period from 1967

to 1975. The %IACS changes in the table

are derived from differences between the

original 1967 DC values of conductivity

(using resistance in ohms, length in

centimeters and area in square

centimeters) and the 1975 values of

conductivity which include grain direction

and stratification corrections. Thus it

is an overall view of changes in certified

values. Other comparisons are made later

on using uncorrected data.

8. Analysis of Changes

If we discount the uncertainties for

the moment and try to determine the com- bined effects of area and resistance

changes on conductivity in %IACS, we can

see that if

%IACS = 172.41 L RA '

the resulting conductivity values should

change inversely as the area and

Table 1

8 Yr. Certified Value

Material (alloy) Area Change Res. Change (DC) %IACS Changes

Copper +0. 127%

Al. HOOF -0. 023 4 Al. 6061 -0. 0293 Al. 5052-0 +0. 005o Al . 2024T4 +0. 094o Al. 2024a

-0. 005o Yel. Brass +0. 0174 Titanium'3

-0. 055% -0 080

-0. 098 -0 003

-0 106 -0 032

-0. 044 -0. 012

0. 000 -0. 149

+0. 105 -0. 072

-0. 103 +0. 018

a 2024T351

b See 75-17L for titanium. Original bar retired-too thin. NOTE: Area change values have an uncertainty of ± 0.08%, the resistance change values

have an uncertainty of <±0.01% and the %IACS value changes, are calculated

from both of the above plus temperature uncertainty and have an uncertainty of

<±0.1%.

125
resistance. Or, stated another way, as the area or resistance increases, the

%IACS should get smaller, all other terms

remaining constant. If the value of

either A or R increases at the same rate

that the other decreases, the effect of

both tend to cancel. An analysis of each

bar follows in table 2.

9. Grain Direction and Stratification

Effect

One reason for the above discrepancy

is that a change was made in the aluminum

set values to correct for grain direction

and stratification. Since the original

bars were cut with the grain direction (or

direction of roll), the %I ACS values tend

to be higher than actual because of the

lower resistance values when measured

along that axis. Tests were performed to determine the magnitude of this effect.

The corrections were calculated to be half

of the difference between "with grain" and

"across grain" values for eddy current

measurement purposes. Since the original

values of conductivity were too high, the

corrective action was to recertify the

%IACS values to a proper lower figure.

Thus, all of the five aluminum alloys had

from -0.02 percent to -0.12 percent IACS

change added algebraically to the most

recently calculated %IACS figures

depending on the particular alloy.

Stratification corrections were generally

about one-third the value of the grain

direction corrections and, except in the case of aluminum HOOF, had the opposite

sign, tending to make %IACS figures higher

in conductivity values. Taking all of

these factors into consideration, we then

find the following result in table 3a and

3b.

10. Comparison of Original and Latest Data

The matching of magnitudes and direction now indicates the actual changes

occurring in the primary bars. A comparison of the uncorrected bar changes

are the changes due only to the area and

resistance changes follows in table 4.

It is significant that the errors due

to grain direction and stratification have

changed originally assigned values by as much as 0.12 %IACS. These corrections were not obtainable at the start of the

program, but are now figured into the

values of the primary bars adding to the

credibility of the certified secondary

standard values. More will be said about

secondary standard certification methods

later in this paper.

11. Additional Primary Bars (75-17 L)

In 1968, a review of the Boeing

Company's requirements resulted in adding

eight additional primary standard con- ductivity bars to the original eight bars.

(The original thin titanium bar was not retired until 1970.) Most of these new

bars were in the lower end of the con- ductivity spectrum because of the large

amount of research being devoted to titanium fabrication in the SST (supersonic

transport) project at that time. Three

bronze bars nominally 6.89 , 8.7 4 , and

16.

6

4 %IACS plus five titanium bars nom- inally 0.97 3 , 1,00a, l-056 , 1.23s, and

3.624 %IACS were processed and calibrated

as described in ISA paper 68-550. A similar error analysis was performed on the 75-17L bars and showed a maximum A%

of %IACS value of -1.14a.

Table 2

Material (alloy) Area. Res. Calc. Change 8 Yr. Cert. Value Change

Copper Lower Lower

Al. HOOF Higher Lower

3

Al. 6061 Higher Lower

3

Al. 5052-0 Higher Lower

3

Al . 2024T4 Lower Lower

Al. 2024 351 Lower Lower

Yel. Brass Higher Higher

30pposite effect than predicted.

126
Table 3a

Pri. Bar

Material

%IACS

AC

a %IACS

corrections

%IACS

DC(new)

Copper

Al. HOOF

Al

Al

Al

Al

6061

5052-0

2024-T4

2024T351

Yel. Brass

101.2

60.16

8

0(est.

)

-0.06.

41.57

35.52,

30.60^

29.83t 27.24,

4 -0.06c

-0.02-

-o.nj

-0.03-

0(est.)

101.2

60.23

0 41.64,

35.54~

30.72

29.87,

27.24,

0

^IACS corrected values are certified from 60 kHz to 200 kHz after

adjusting for grain direction and stratification effects.

Table 3b

Pri. Bar Material %IACS DC (Orig) %IACS DC Diff.

Copper

Al. HOOF

Al

Al

Al

Al

6061

5052-0

2024-T4

2024T351

Yel. Brass

101.

2g

60.17

] 41. 60,

35. 53

] 30.75,

29.88

f 27.23,

-0.08

+0.06

+0.04

+0.02

-0.03.

-0.01

+0.01

Table 4

(Table 3) (Table 1)

Pri. Bar Material Uncorr. Changes Area Res. Changes Diff. %IACS a

Copper -0. 08%IACS +0 07%IACS 0 01 Al. HOOF +0. 06 -o. 12 0. 06

Al. 6061 +0. 04 -0. 13 0. 09

Al. 5052-0 +0. 02 -o. 04 0. 02

Al. 2024 T4 -0. 03 +0. 09 0. 06

Al. 2024T351 -0. 01 +0. 10 0. 09

Yel. Brass +0. 01 -o. 09 0. 08

a Evidence of the repeatability of readings taken with the facility.

127
12. Filling in Some Gaps in %IACS

Standards Values

Even though the low end of the con- ductivity spectrum seems to be quite

complete, especially in the titanium

range, it was necessary to add a new bar

so that eddy current meters having a range

of 0 to 3.5 %IACS could have a meaningful

calibration. A review of the titanium

standards shows the highest value to the

3.6 %IACS which is off scale, and the next

lower value is 1.2 %IACS which is too far

down scale to be significant when used by

itself or with lower values in %IACS. A new bar having a value of 3.3 7 %IACS has

been fabricated and put into the set of 19

total located in the transfer oil bath

previously shown in figure 11.

The portion of the aluminum con- ductivity spectrum existing between the

41.6 %IACS and 60.2 %IACS standards

required at least 2 more standards to produce better results for eddy current

meter calibration and for curve-fitting

the data better as explained later on in

this paper. Unfortunately, although two different alloys of aluminum, 6061-0 and

2024-0 were processed, the values turned

out to be within about 0.2 %IACS of each

other at about 47.5 %IACS rather than the

nominal 5 percent difference expected from

table of physical properties used for the

selection process.

As a result of previous experience

and analysis, both aluminum bars were

selected in the -0 condition for least

stratification and were then cut at 45° to the direction of roll, effectively negating

the effect of grain direction. This means

that the AC values assigned to each bar

will be the same as the DC values, and no corrections are required for these 2 potential sources of error in the transfer

from DC to AC values. The fourth area which required at least one additional

primary standards bar was between the 60.2

and 101.1 %IACS standards. A conducting

bronze bar was fabricated in the same manner as previously described in the

references, and now gives an additional

certified conductivity value in that area

at 85.39 %IACS. None of the 4 new bars were tested for permeability, based on the

results of the original tests on the first

16 bars, which showed negligible effects

from permeability, except copper nickel

bronze.

The results of the calibration of the

four new non-ferrous conductivity bars are

as follows in table 5.

13. Improved Accuracy Transfer to Secondary Standards

The original method developed to determine values of secondary conductivity

standards utilized an a-c impedance

bridge, modified to include a guard

circuit, lift-off compensation and various

probe compensation capacitor settings. The

frequency was fixed at 100 kHz ± 1.0 Hz and power entered the bridge circuit at a voltage somewhat less than 10 volts rms. An a-c null detector indicated the bridge

balance condition and a Wagner ground was included for precise and repeatable

detector balance. See figure 12 for the

schematic of the bridge circuit.

The initial method of experimenting

with slope curves in pFAIACS values vs. %IACS, with changing probe compensation

capacitance values in segmented areas and

with weighted averages for determination

of secondary standard conductivity values

was adequate when comparing like or very

close values of conductivity. It failed

Table 5

Material (alloy) Conductivity Temp. Coeff

.

of Resis/°Ca

Conducting Bronze 85.39 %IACS 0 003

2 Al. 6061-0 47.64

3

0. 003

2 Al. 2024-0 47. 46-, 0. 003

3 Titanium 55 3.37

6

0. 003

6

The temperature coefficient of resistivity per °C is accurate to ±0.0002

between 15 and 35 °C also called TCF or temperature correction factor.

128
to deliver the expected accuracy, however,

when the known values were over two percent different in %IACS from the

reference standard values. Deviations

were discovered when one primary standard

bar was used to verify another primary

standard bar several %IACS different from

the first. The ensuing investigation

showed that the entire premise on which

transfer of primary bar accuracy to secondary standards was based, with

allowance for bridge errors, was somewhat

less accurate when the two standards,

reference and unknown, differed by more

than 2 percent. With some of the

secondary standard nominal values several

percent away from closest reference

standard, certifications within ±0.35

%IACS of the stated values required

re-examination.

CONDUCT' /IT* SPiOCt

Figure 12.

14. Curve Fitting Method

After analyzing several different

approaches, it was decided to segment the

conductivity spectrum into six parts and

use curve fitting techniques, with an H-P

65 to determine the unknown values based

only on the pF values at balance. Only

one fixed probe compensation setting was

used. Corrections to each segment, depended upon how close to the primary

standards values the unknown standard was

and corrections were automatically

applied. The results were well within our anticipated values of accuracy when

calibrating one primary standard against

any other primary standard in a particular

segment.

The six segments and the method of curve fitting applied is shown below.

15. Development of Curve Fitting

Method Details

Interpolation formulas were derived

using curve fitting techniques based, in

part, on programs in the Stat. Pac 1 of

the Hewlett Packard HP-65 programmable

calculator. Since it had been previously

observed that if the bridge readings in pF were plotted with the %IACS on log- log

graph paper, the resulting points were nearly in a straight line; logarithms of

the bridge readings and %IACS values were

used in the curve fitting process. An attempt to determine a single overall

formula for the entire range from 0.96

percent to 101.2 %IACS showed that errors would be too large. Therefore, the %IACS

values were divided into segments as

fol lows:

(1) 0.96% to 1.23%

(2) 1.2% to 3.6%

(3) 6.8% to 27.2%

(4) 27.2% to 41.6%

(5) 35.6% to 47%

(6) 41.6% to 101.2%

Segment No. Conductivity Limits %IACS Curve Fitting Method

1 0.96 to 1.23 Mod. Pwr. Curve

2 1.2 to 3.6 Mod. Pwr. Curve

3 6.8 to 27.2 Linear Regression

4 27.2 to 41.6 Mod. Pwr. Curve

5 35.6 to 47 Mod. Pwr. Curve

6 41.6 to 101.2 Mod. Pwr. Curve

129
!

A different formula was derived for

each segment. Linear regression proved to give an acceptable fit for the data for

segment (3); the other segments were curve

fitted using a modification of the power

curve. These formulas, as applied here, are as fol lows:

Linear regression:

log %IACS = A

Q + A] log pF

Power curve (modified):

log %IACS - log pF 1

where A 0 , A 1( a, and b are coefficients

determined by the curve fitting process

and "pF" is the corrected bridge reading

in pF.

16. Programming the Calculator

These coefficients are then incorporated into HP 65 programs--one for each

segment—which are used to determine

conductivity of standards tested. In some

cases, additional corrections are incorporated to reduce errors at points on the

interpolation curve close to the

conductivities of the standard bars.

Since the %I ACS is determined by the

formulas in terms of only one variable,

the bridge reading in pF, no "standard

reading", as such, is necessary. In practice, however, a bridge balance is made for a standard close (in %IACS) to

the value of the "unknown" to account for

any drift in the capacitance elements in

the bridge. The difference between the

standard reading taken at that time and

the standard reading used to determine the

interpolation curve is then used to shift

the unknown reading by a like amount

before applying the interpolation formula.

The HP-65 curve fitting programs used

also provide an additional parameter r

2

, or "goodness of fit." It was found that

this number must be quite close to 1; 0.999 or greater in most cases to give

satisfactory results. This is probably due

mostly to the scale compressing effects of

logarithms.

17. Secondary Conductivity Standard

Calibration Improvements

A new oscillator with improved output

voltage and better stability characteristics which can now deliver a steady

10 volts rms to the bridge circuit at 100

kHz ± 1 Hz has replaced the original

oscillator. The results have been much

more consistent readings from the primary

standards bar as well as a slight improvement in detector sensitivity due to somewhat higher voltage. See figure 13 for a view of the probe and the bridge

circuit control console.

Figure 13. 100 kHz conductivity transfer

bridge and probe.

When coupled with the new curve fitting techniques, the accuracy improvement in actual bridge reading values

reduces the uncertainty of %IACS accuracy

limits from an estimated +0.15 percent to approximately +0.07 %IACS. Adding this

uncertainty to the +0.2 %IACS, which is now thought to be quite conservative based

on historical performance evaluation, we

feel confident of the stated ±0.35 %IACS,

or 1 percent of reading accuracies,

whichever is less, assigned to secondary

non-ferrous conductivity standards

produced by the Boeing Metrology

Laboratories. Figure 5 showed some types

of secondary standards in their containers

after being calibrated against the primary

bars in the transfer oil bath.

18. How Good Are Eddy Current

Meter Standards?

From what has been discussed

previously, it can be seen that unless some

130
unknown factor has not been considered, the

Boeing secondary non-ferrous conductivity

standards which are categorized to be

working standards for conductivity, are

well within the ±0.35 %IACS or 1 percent of

value uncertainty assigned to them.

Periodic recal ibration and recertifi cation

of the Boeing primary standard bars and the

annual in-oil recertif ication of the

secondary standards using the 100 kHz

conductivity bridge transfer oil bath shown

in figure 13 will keep the secondary

standards within the assigned accuracy

1 imits.

It has been Boeing policy to resurface

secondary standards which are received for

recal ibration in such condition that liftoff errors can exist as a result of excessive wear. Some slight drift characteristics have been observed in secondary

standards over the 10 year period, but

recerti f ication keeps them well within

their uncertainty limits for the one calendar year cycle assigned to them inhouse. Commercial customers usually

observe longer cycles to suit their needs

or internal cycle periods.

Using these secondary conductivity

standards serves to guarantee the accuracy

of direct reading eddy current meters by

verifying the scale tracking in the area of

interest. Although, for example, several

scale points in the most frequently used

aluminum conductivity range, 28 to 60

%IACS, are compared to the standards in

that range, this does not assure the

accuracy of scale indication outside of

that range. If the entire scale is to be

utilized from the low or titanium range up through copper, certified secondary con- ductivity standards covering the entire

range from 1 to 100 %IACS should be used.

Without such standards the indicated

values of either direct or indirect eddy

current meters are questionable.

Several manufacturers are now in the

business of providing certified conductivity standards for use with eddy current

meters. A few of the types produced by

the Boeing Company were shown in figure

5. Several other configurations have been

made to satisfy internal requirements, but

basically the accuracies and non-ferrous

materials are the same as with the stan- dards forms.

Error Analysis

Remarks

Primary Standard Bars (used to calibrate secondary coupons)

Component Max Expected Error

DC Ref. Resistor

Double Ratio Set

Galvo Readability

Thermometer

Thermocouples in bath

DC Sys. Instability

Conductivity Material

Stratification Error

Grain Direction Error

Repeatabi 1 ity

Total RSS error = ±0.12% of reading in ohm cm @ 20 °C

(cert to ±0.20 % IACS or 0.5% of reading, whichever is less at 20 C)

±0. 005% Traceable to NBS Thomas 1 ohm

±0. 001%

±0. 01% At maximum sensitivity

±0. 012% 19° to 21 °C

±0 0016% Galvo reads opposing T.C. ±0 04% Temperature of bath oil

±0 054% Non-homogeneity

NA Corrections Made

NA Corrections Made

<±0 1%

Secondary Standard Coupons

Component Max Expected Error

±0.12% of reading

<±0. 10% of reading

<±0. 10% of reading

±0.05% of reading

±0. 10% of reading (max.

)

±0.05% of reading

Total RSS error = ±0.22% of reading . (Cert, to ±0.35 %IACS or 1% of reading, whichever is less at 20 C)

Primary bar basic error

100 kHz bridge error

Probe error (position)

Curve fitting error

Aging and wear (1 yr. cycle)

Temperature uncertainty

131
19. Conclusions

The preceding error analysis indicates that the certified accuracies

assigned to the primary standard

conductivity bars for a 2 year

re-certification cycle and the secondary

standards for a 1 year cycle are reasonable. Coupled with this, our experience has shown minimal differences

in the primary bar values due to changes

in solid state structure and other aging

factors.

The fact that all the measurements

are basic parameters which have been in

existence for decades gives us confidence

in our ability to provide indirect traceability to NBS basic standards of

resistance, length and temperature.

Adherence to good machine shop practices,

careful screening of materials,

utilization of the latest measurement

techniques and, above all, careful

attention to minute details in both

constructing and measuring these standards

has incorporated a high order of built-in

accuracy.

At present, NBS in Gaithersburg,

Maryland, is preparing to construct

non-ferrous conductivity standards. They

indicate that comparisons with these

standards are still about a year away. Discussion with NBS personnel indicate

that an approach similar to the one used

at Boeing will be used by them both in

construction and in measurement of their

conductivity standards.

The only additional research proposed

in this field at Boeing is to build about

four more primary standards bars this year

to increase the accuracy of the curve

fitting technique in some of the segments

where the spread between any two of the

primary bars is larger than deemed suitable.

References

[1] Jones, A. R. , Bulk Conductivity Measurement Standards, presented at ASTM

Annual Meeting, Boston, Mass. , June

1967.

[2] Jones, A. R. , Non-Ferrous Conductivity Standards, presented at ISA

Annual Meeting, New York City,

October, 1968 (68-550).

[3] Jones, A. R. , Error Analysis of Non- Ferrous Conductivity Standards, presented at the ISA International Conference and Exhibit, Philadelphia,

Pennsylvania, October 1970 (70-613).

[4] The Boeing Company D6-4813, Material/

Temper Identification - Aluminum

Al loys.

[5] ASTM Designation B193-65, Resistivity

of Electrical Conductor Materials

(Standard Method of Test).

[6] NBS Copper Wire Table, No. 31, p. 75.

[7] Libby, H. L. , Introduction to Electromagnetic Nondestructive Test Methods.

[8] Jones, A. R. , Non-Ferrous Conductivity Standards - A Ten Year Review,

presented at ISA International Conference and Exhibit, Houston, Texas,

October, 1976.

[9] Jones, A. R. , Eddy Current Meter

Standards, How Good Are They?,

presented at ASNT Spring Conference,

Phoenix, Arizona, March, 1977.

Discussion

Question (Dr. Mc Master): I have found

there are measurable eddy currents up to three times the diameter of the coil.

Every time you measure something smaller

than that, you are clipping the corners. Have you found much error due to the ratio

of coil diameters, or have all coil

diameters you have tested been so small

that you are nowhere near the sample edge?

(Mr. Jones): You are talking about edge

effects?

(Dr. Mc Master): Yes.

Answer (Mr. Jones): We measure in the

center of the secondary standards with a small coil and try to get somewhere near

the middle of the primary bar. Also, we always measure on the same spot on the

primary bar.

132
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977. Issued

January 1981.

NBS EDDY CURRENT STANDARDS PROGRAM

George M. Free

Electrical Measurements and Standards Division

National Bureau of Standards

Washington, DC 20234

1. Introduction

The present goals of the NBS eddy

current program are twofold: the creation

of an electrical conductivity calibration

service for non-ferrous metal standards,

and the development of non-ferrous metal

Standard Reference Materials to be issued

as conductivity standards. The calibration

service will provide measurements of the

electrical conductivity of standards sent

to NBS by industry and issue reports on these standards. The Standard Reference

Material Program will make available to industry electrical conductivity standards

suitable for use in calibrating eddy

current instrumentation.

In order to establish the NBS

electrical conductivity calibration

service instrumentation and methodology

must be developed in several areas [I] 1

. Primary conductivity standards consisting

of long metal bars are being developed

which will be measured using dc techniques

to determine the conductivity of the

material. Using eddy-current techniques,

customer conductivity samples will then be

compared with the primary standards to arrive at the conductivity of the sample.

Work has been developing along two

lines, design and construction of dc

measurement apparatus, and design and

construction of ac measurement apparatus.

These two measurement systems will be

discussed in more detail.

2. The DC System

Since eddy current measurements are made at a point, or over a small area, the

NBS dc measurement is designed to determine conductivity at a specific point

From the relation J = aE in which J is the

current density, o the electrical conductivity and E the electric field, the con- ductivity of a uniform bar may be written

a =

L A = cross sectional area of

RA material

L = distance between potential

contacts

R = resistance

if R and A are considered to be average

values over the length L. If the bar is

only slightly irregular, this relation

will hold if a, dR/dx, and A are considered to be slowly varying functions over

the length of the bar.

a =

1/A = 1

1 dR dR

dx dx

where A is the cross-sectional area at a specific point on the bar and dR/dx is the

slope of the resistance vs. length curve

at a specific point. In approaching the

determination of conductivity in this manner, two sets of information are nec- essary. A mapping of the cross-sectional

area along the length of the bar and a mapping of the resistance of the bar. The

two sets of data will then appear as in

figure 1

.

To determine a, the slope of the re- sistance curve at a point will be calculated and the area of the bar at that

point will be determined. Dimensional

measurements of the bar will be done by

another group within NBS. The dc measurement of resistance will be done using a current comparator as described below.

The operation of the current comparator is shown in figure 2. A current is

1 Figures in brackets indicate the literature references at the end of this paper.

133
ALONG BAR Figure 1. Determination of (dR/dx) and

(A) at a specific cross-section of metal

bar.

CIRCUIT 2uCIRCUIT 1

UTAL

BAR

D.C. SLAVE „ 1

1

1 D,C. supply

nm r rnV>

uuuj|

Figure 2. Current comparator schematic

with metal bar in place.

established in circuit (1) by the main

power supply. The primary and secondary

currents separately pass through a number

of turns wrapped on a common core, the

turns being wound in opposition; a flux sensor detects any residual flux in the core and adjusts the slave power supply so that zero flux exists in the steady state. The I N = I N . The turns ratio N

X X s s s can also be varied to achieve a null at the

detector. At the point of balance I R =

I R or R = R M /N . Then tlfe fin- known

5 resistance is

s l&ow^ in terms of a standard and a turns ratio.

During the resistance measurement,

the metal bar will rest on a modified op- tical bench, as in figure 3. Current will

be injected at (A) and leave at (B); the

potential contacts are at (1) and (2).

Potential contact (1) will remain fixed

while (2) will be varied along the length

POTENTIAL CONTACTS m (2) C) <2">

\ [ I I PETAL BAR

, _J 1 III I »

, '"

I I II I I ^

/ OPTICAL BENCH _^

Figure 3. Motion of potential contacts

along length of metal bar.

of the bar. The separation distance between potential points will be measured by

a laser interferometer. Thus, a series of

measurements of resistance can be made

along the bar with a minimum distance of

separation of 1 cm. The total length of

the bar will be approximately 1.5m with a cross-section of 50 mm x 6 mm.

The measurement will be done in an

oil bath which is stable to about ±3 x

10

3 °C. The range of temperature possible in the bath is 18 °C to 30 °C. With

temperature, dR/dx, and cross-sectional

area known, the conductivity can be calculated. The bars will then become the

primary standards of conductivity at NBS.

3. AC System

The eddy current measurements will be

done on a bridge as seen in figure 4. The

major components of the bridge are the

three inductive voltage dividers, a variable frequency power supply and a phase

sensitive detector. The bridge is designed for use in three general ways. In the positions of the two test coils on the

bridge diagram, the following may be

placed:

(a) Coil and capacitor. Used to calibrate inductance in terms of capacitance standards.

(b) Two coils used to make the measurements in an absolute sense. One coil

will be in air while the other coil

will be placed on the metal sample.

(c) Two coils used to make the measurements in a relative sense. Both

coils will be placed on the same piece of metal for a zero reading,

then one coil will be placed on the

unknown sample. The change in con- ductivity will be determined from the

change in the bridge balance

conditions.

TEST COILS

NETWORK Figure 4. Eddy current bridge

134
The actual conductivity of an unknown

will be obtained by interpolating the ex- perimental results for measurements on two

primary standards and the unknown, using

primary standards having conductivities

above and below the unknown.

4. NBS Services

Reports on the calibration of a cus- tomer's sample are envisioned as follows.

Besides a written report, a graph will be

given as seen in figure 5. Conductivity

will be shown as a function of

frequency and as a function of temperature

for a specific range of frequencies. The

total uncertainty assigned to the test

will be a function of the metal sample

that is being calibrated, i.e., its

uniformity. For a uniform metal sample the

total uncertainty should be about 0.1

percent IACS.

5. Other Areas of Study

Coils are being constructed using the

relations derived by Dodd et al . [2].

Values for the impedance of a coil on a conducting flat surface have been calculated. These results are being used to achieve optimum performance in the coils

constructed, i.e., maximum sensitivity

to change in conductivity and minimum

change in impedance for lift-off. The

relations will also be used to establish

the theoretical shape of the impedance

curve between points of known dc con- ductivity, i.e., those points for which

primary standards are available. An attempt will be made to correlate

measured coil parameters with the theoretical values. If the agreement

between actual measured values and theoretical calculations is sufficiently

close, a quasi-absolute determination of

conductivity can be made.

IACS

-i 1- -i 1 1 1 h

FREQUENCY Hz

Figure 5. Information on conductivity

sample to be included in test report.

Although plans for constructing the

Standard Reference Materials are not yet

finalized, there are several directions

that could possibly be followed. First is

the use of powder metallurgy or some other

metallurgical technique to guarantee that

the sample is indeed uniform. A calibration report as above would be issued

with the sample. Another possibility is to use the most common metals and alloys

for standards. If common metals are used,

the need for temperature corrections would

be eliminated since the metal to be tested

would be at the same temperature as the

standards used to calibrate the eddy

current meter.

A second experiment that has been

briefly investigated but which will be

pursued further is based on the theorem of

Van der Pauw [3]. This relation is com- monly used in semiconductor technology to determine the dc resistivity of a sample

of uniform thickness.

In this theorem,

relation is derived:

the following

^nRid/p + e

-'nR 2 d/p _

1

Resistivity becomes a function of two re- sistance measurements, R x and R 2 , and a measurement of d the thickness of the

sample. The advantages are obvious.

All of the above work will be directed towards the goals stated at the

beginning of the paper, the establishment

of precise standards of electrical con- ductivity which are usable by industry.

The present goal of the program is to have the electrical conductivity calibration service available in the Fall of

1978. The first SRM's will be issued

in 1979.

References

[1] Jones, Arthur R, "Electrical Conductivity Measurement Standards," Pre- sented at ISA Conference, Boston,

1970.

135
[2] Dodd, C. V., Deeds, W. E. , Luquire,

J. W. , Integral solutions to some eddy current problems, International

Journal of Nondestructive Testing, 2>

29-90 (1969).

[3] Van der Pauw, J. J., A method of

measuring specific resistivity and

Hall effect of discs of arbitrary

shape, Philips Research Reports, 1_3, No. 1 (February 1958).

Discussion

Question (Mr. Wehrmeister): I am curious

to know what sort of tolerances you are putting on your readings in terms of

percent IACS? And what tolerance is

industry working towards?

Answer (Mr. Free): In this experiment, we

are hoping to achieve an uncertainty of .1 percent IACS. By tolerance, do you mean

the uncertainty in measuring conductivity

in a sorting operation?

Question (Mr. Wehrmeister): How accurately do they have to obtain a reading or a measurement?

Answer (Mr. Free): As I understand it,

when running a characteristics test on incoming metal, it is around 4 or 5 percent.

Comment (Mr. Jones): Usually, the precision should be plus or minus .5 percent,

IACS. For example, if you have an allowed

band of 31-1/2 percent to 33-1/2 percent,

you would have to hold that to 32 percent

or 33 percent, but 15 percent would be

used up in your measurement uncertainty,

considering the conductivity standards

being used.

On the primary bars, we try and get

to .2 percent plus or minus half percent

reading, whichever is less.

Comment (Mr. Lagin): One thing which I think industry would like to see come out

of NBS in the future is a standard representing various radii of curvature.

Usually we have to take conductivity mea- surements on curved surfaces, and since

the probe has finite dimensions this

introduces error. There is a large percentage of error when the parts have a radius of curvature of less than half an

inch. The conductivity measured could be

in error by as much as 10 percent.

Question (Mr. Wehrmeister): What type of

geometry in test samples will you be ac- cepting. Will they be flat, or can I send

you a piece of tubing?

Answer (Mr. Free): The samples, at least

to start out with, will be flat, geometrically a flat sample.

Question (Mr. Jones): When you are making

your resistance measurements, what type of

spacing will you have between the

potential probes?

In other words, say you start near

the top, do you take ten readings and the move a quarter of an inch, and then take

ten more readings to get the uniformity

across the width of your standard bar

which you said was two inches?

Question (Mr. Free): Do you mean to get

the mapping of the cross-sectional area, or the mapping of the resistance?

Answer (Mr. Jones): The mapping of the

resi stance.

Answer (Mr. Free): Initially, there will

be two measurements.

Question (Mr. Jones): Somewhere near the

middle?

Answer (Mr. Free): Two points, somewhat

off center, but on either side of the

center.

136
National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy Current Nondestructive Testing held at NBS, Gaithersburg, MD, November 3-4, 1977 Issued January 1981. ' ucu

ELECTROMAGNETIC THEORY AND ITS RELATIONSHIP TO STANDARDS

Arnold H. Kahn and Richard D. Spal 1 Center for Materials Science

National Measurement Laboratory

National Bureau of Standards

Washington, DC 20234

1. Introduction

The interpretation and derivation of

the maximum information from electromagnetic testing and the optimum design

of testing methods depends on principles

found in classical electromagnetic theory. The value of an approach based, in

part, on a theoretical grounding is well

demonstrated in the discussion by Libby

[l] 2. In the development of standards,

the application of theoretical analysis

can be used to base the standards, at least in part, on physical laws. We hope

that discussions at this meeting will

suggest areas where theoretical efforts

may be profitably undertaken.

As one of the NBS activities in

electromagnetic NDE, we have performed

analyses of the nature of eddy current

distributions in the vicinity of a crack

in conducting material. The purpose of

these investigations is to provide predictions of the changes of signals in

testing apparatus due to cracks, or alternatively to aid in the characterization of defects.

Literature searches on this subject

have yielded only a few theoretical

treatments of the modification of eddy

currents due to the presence of defects.

The treatment of Burrows [2] and Dodd, et

al., [3] considers the perturbation of

the eddy current pattern by an ellipsoidal inclusion. However, in this

treatment the inclusion had dimensions

which were small with respect to the

electromagnetic skin depth. However, for

greatest sensitivity of detection the

frequencies must be such that the skin

depth is of the same order of magnitude

as the dimensions of the obstacle. In this contribution to the Workshop, we report on calculations of the eddy cur- rent and impedance changes associated

with a surface crack in a plane slab and

on a cylinder. In the case of the cylinder, impedance diagrams are given, and

the modifications due to the crack are demonstrated.

2. Crack on a Plane Slab

Our model for a crack [4] assumes that there is a long, thin gap in the

conducting material. We treat the gap as preventing the flow of any normal com- ponent of electric current density. The

magnetic field for which we perform the

calculations is applied tangential to the

surface and parallel to the crack. The

geometric configuration is shown in figure 1. In the figure, the width of

the crack is exaggerated so as to show

the unrestricted penetration of the

applied a.c. magnetic field, H

Q

e

1U)t

, into the crack. The lines with arrow

heads indicate schematically the direction

and path of the current at some instant

of time. The solution to this problem is

moderately difficult and much can be

learned by breaking it into two component

problems, the corner and the tip, as shown in figure 2. These can be solved

exactly. The results may then be com- bined to give the results for a crack

provided its depth is greater than four

skin depths.

The equation to be solved is [1,4]

(V2 +k2 )H = 0 , (2. 1)

department of Physics, University of Pennsylvania, Philadelphia, PA 19104.

2 Figures in brackets indicate literature references at the end of this paper.

137
Figure 1. Schematic drawing of eddy

currents in the vicinity of a surface

crack in a slab of conducting material.

The a.c. magnetic field applied at the

surface is normal to the figure and

uniform in space.

where the complex propagation constant,

k, is

k = (iouiM) '

. (2.2)

In the above equations, H is the field

inside the conductor, a is the electrical

conductivity, w is the angular frequency

of the applied field, and u is the permeability of the conductor. At the

surface of the material, and in the crack

gap, the field must take on the value of

the applied field, H Qe

10

. In terms of

the electromagnetic sKin depth, 6, given

by

6 = (2/ou)u)

,/£

, (2.3)

the propagation constant may be expressed

as

,

k = (l+i)/6 . (2.4)

a) INFINITE CRACK

Y

7.

In order to calculate changes of power

dissipation and energy stored, it is not necessary to map the field; we need only

know the current density at the surface,

which, in turn, depends on the normal

derivative of the field at the surface.

The results are conveniently expressed in

terms of the complex Poynting vector,

S = l/2(ExH*) ,

b) RIGHT-ANGLED CORNER

V

Figure 2. Two solvable problems related

to the crack of figure 1. In the

infinite crack, a magnetic field,

H

Q

e

1ujt

, is present in the gap as far

as the tip. In the right-angled corner

problem, the field is applied to the

boundary, parallel to the corner. Arrows indicate the current flow.

where E and H are the electric and magnetic fields at the surface of the

material. The real part of S gives the

power dissipation per unit area and the

imaginary part gives the energy stored

[4,5]. (In the next case, the cylinder

with a crack, the Poynting vector leads

directly to the complex impedance.)

In figure 3, we show the normal com- ponent of the Poynting vector, S, as a function of distance from the tip of a deep crack. The values of S are nor- malized to S 0 , the value appropriate to plane surface in the absence of a crack. We see that the greatest dissipation and

reactance changes occur near the tip and

that at a distance beyond 1.56 from the

tip fields are "back to normal." A similar situation occurs at the corner, as

138
is seen in figure 4. The loss at the corner vanishes, but the "range" of the corner is about 2.56. The losses at the

1

lm S

0.0 0.5 15 DISTANCE FROM TIP. */8

Figure 3. Plot of the normal component of

the Poynting vector on the surface of an infinite crack at a distance, x, from the

tip. In the plot the Poynting vector is

normalized to ReS0 , where S 0 is the

Poynting vector for plane surface in the

absence of a crack. Distance is in units

of 6, the skin depth.

Figure 4. Plot of the normalized Poynting

vector, S', on the surface of a square

corner, at a distance, x, from the

corner. Distance is in units of 6, the

^kin depth.

tip are greater than those of equivalent

flat surface, while those at the corner

are less. In effect, the eddy currents

take a short cut at the corners and a long trip around the tip. The net dissipation, per unit length of crack, for a crack of depth d, with d > 46 is

(2d-0.786) |H

Q

| 2 /(2a6) . (2.5)

There is an effective shrinkage of the

depth, relative to equivalent flat sur- face, of approximately 0.396. For cracks

more shallow than 46, a direct solution

is necessary. This will be done in the

fol lowi ng section.

3. Surface Crack on a Long Cylinder

In this section, we present the

results of calculations of the eddy

currents in a long cylinder with a radial

crack at the surface. The calculations

were performed for arbitrary depth of

crack, and the results were developed in

terms of an infinite series of trigonometric functions and cylindrical Bessel

functions. The results can be made

visual izable by means of the impedance

diagram.

In figure 5 we show the familiar

impedance diagram for a solenoid enclosing a cylinder (with a 100 percent filling factor) [1]. The plot shows the

imaginary versus the real parts of the

impedance of the coil. A point on the curve corresponds to a particular value

of the ratio of cy linder radius to skin

depth, a/6(= aVou)u/2).

The effects of cracks of varying

depth are now shown in figure 6. Our

first observation is that the presence of

a crack does not change the shape of the curve; it only shifts the position on the curve of each a/6 point. For an initial

attempt to understand the phenomena,

we calculated the impedance curves for

cracks of depth d = 2a, a, and 0.5a. We

have plotted these curves, shifted in space in the figure. Equal values of a/6

on the different curves are connected by

dashed lines. It is evident that the

greatest shift of impedance caused by the

presence of the crack occurs for a/6 ~

1.7, where the curve has a vertical

tangent.
Figure 5. Impedance diagram for a long

cylindrical solenoid with a conducting

core which completely fills the solenoid.

The real and imaginary parts of the

impedance are normalized to ujLo, the

reactance of the empty solenoids.

Points on the plot correspond to particular values of a/6, the ratio

of core radius to skin depth.

These calculations have not yet been

carried out for very shallow cracks

because of slow convergence of the

series. To cope with this difficulty, we have developed as an alternative a variational approach which should yield

greatest accuracy for shallow cracks and

which will overlap the region treated by

the series method. This work is now in

progress.

A provisional estimate for the

effect of the crack can be given. If the

frequency is chosen so that a/8 a 1.7

(near the maximum dissipation), then we expect the shift of the a/6 to be ~ -0.4

times the value of d/a. For these values,

the impedance change is imaginary and

corresponds to a shift of the amount

Figure 6. Impedance diagrams for a long

cylindrical solenoid with a conducting

core which contains a radial surface

crack. Impedance curves are plotted,

as in figure 5, for three selected

depths of crack. The dashed lines

schematically indicate the shifts of

a/6 points as the crack depth, d, is varied in the calculations.

References

[1] Libby, H. L. , Introduction to Electromagnetic Nondestructive Test

Methods (Wiley-Interscience, New

York, 1971).

[2] Burrows, M. L , "A Theory of Eddy

Current Flaw Detection," Dissertation, University Microfilms, Inc., Ann Arbor, Michigan (1964).

[3] Dodd, C. V., Deeds, W. E. , and

Luquire, J. W. , Int. J. of Non- destruc. Testing 2, 29 (1969).

[4] Kahn, A. H. , Spal , R. , and Feldman,

A., J. Appl. Phys. (to be published).

140
[5] Collin, R. E. , Field Theory of

Guided Waves (McGraw-Hill Book Co.,

New York, 1960).

Di scussion

Question (Mr. Libby): I understood you

to say that you found no difference in

the shape of the curve for these con- ditions?

Answer (Mr. Kahn): We found slight differences which we believe are computer

error.

Comment (Mr. Libby): I am rather sur- prised at this. What it indicates, looking at the curve, is that the effect of

the crack would be the same effect obtained by lowering the frequency.

Answer (Mr. Kahn): Right.

Comment (Mr. Libby): I would expect when

the currents are flowing in the usual way

about the crack, there would be some component that tells the coil that we are dealing with a bar having a smaller

radius. I am just wondering why this is not the case.

Question (Mr. Kahn): You are saying it

is not?

Answer (Mr. Libby): I would expect the

effect to show up in the size of the

curve. I would expect that where you

have a curve like the one shown that with

a smaller diameter it would shift down

and the curve would be in a different

place on the complex plane. This is what

I would expect.

Comment (Mr. Kahn): I normalized the curve, it is normalized to the inductance

of the empty coi 1

.

Question (Dr. Birnbaum): Perhaps I did

not understand. You based the calculations on what assumption, that the skin

depth was much larger than the crack

depth?

Answer (Mr. Kahn): No, it is arbitrary.

The whole curve marks out an arbitrary

range of the skin depth from zero to infinity.

Comment (Dr. Birnbaum): Well, then I agree with Libby, it defies intuition.

One would expect that as you vary the

frequency, the skin depth becomes the same order as the crack length, and then

when it becomes larger you would get some more drastic differences in impedance.

Comment (Mr. Kahn): No, I find it just moves it on the curve.

141

National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy Current Nondestructive Testing held at NBS, Gai thersburg , MD, November 3-4, 1977. Issued

January 1981.

DISCUSSION

NEW SCIENCE DIRECTIONS AND OPPORTUNITIES

CONCLUDED FROM WORKSHOP SESSIONS

Robert E. Green, Jr. Johns Hopkins University

Department of Mechanics and Materials Science

Baltimore, MD 21218

In the first talk of the Workshop, by

Dr. Mc Master, the point was made that much

of the eddy current instrumentation and

test methods existing today has not

advanced beyond principles established in

1830. Eddy current development work has

used the new technologies available only

to refine and improve systems that have

been used for many years. New approaches

to basic problems are almost nonexistent.

Beyond this, much of the information that

is present in an eddy current test is

completely ignored.

Listed in figure 1 are general areas

in which research and development work

could be done. These areas are meant to

be independent of standards. They are areas which could be important for

scientific as well as technological

progress. Your comments on these areas,

or additions you might make are important.

I. Specimens

Ferrous

Nonferrous

Homogeneity

Anisotropy

Geometry

Multilayer

II. Coils

Geometry

Orientation

Arrays

Other Detectors

Computer Involvement

III. Displays

Go, No Go, or Meter

CRT

Other Imaging Devices

IV. Environment

Thermal

Radiation

V. Other Methods

Microwave

Triboelectric

Figure 1. General areas for research and

development work.

Although I have not listed the various items in any special order, we definitely have to consider the material

aspects of the part to be inspected. In a research laboratory, we can tailor make

materials so that a- given physical property can be varied in a systematic manner

while holding other properties constant.

Thus, in the laboratory, we can use specially selected materials to test the

sensitivity of eddy current instruments to

a variety of material parameters. On the

other hand, in field eddy current testing,

we have to work with materials which have

usually been optimally designed to possess

desirable mechanical properties, but which

may possess extremely complex electrical

and magnetic properties and, therefore,

present special problems for eddy current

testing. Thus, we must consider, as has

been pointed out several times, whether

the material is ferrous or nonferrous

and probably even more critical with

respect to eddy current measurements is the inhomogeneity of real structural

materials.

In my own opinion, a good eddy cur- rent tester is a fairly sensitive scientific device, and therefore it measures

both changes in electrical conductivity,

magnetic permeability, and geometrical

effects. Alloy type materials which are

143
most commonly used, have more than one chemical or elemental constituent, and

these often segregate in different con- figurations. Therefore, you will not get

a constant eddy current reading no matter

what parameter you are measuring on your

standard piece, if the eddy current detector is sufficiently sensitive. I am not

really clear in my mind how this is going

to be overcome in general, but I think

that specimens are an area for consideration.

Multi- layer types of materials are also important. There was some work re- ported, and composite materials were briefly mentioned. Here, we are dealing

with extreme inhomogeneity of some types

of materials to be inspected. If a scientist was concerned with materials, I think this first section would be of great

interest to him in not only making conductivity measurements, permeability measurements, etc., but perhaps residual stress

type measurements or other types.

The geometrical problem was pointed

out by several people. People would like

to be able to inspect various configurations and parts of present configurations that cannot presently be done.

Most of the other areas I would think

would appeal more to electrical engineers,

electronic engineers, or physicists, or people with that bent.

With respect to coils, Professor Mc Master brought up a number of the parameters in his talk the first day. The

different geometries of coils, using a coil of triangular or ellipsoidal shape,

might be of interest. The orientation of

the coil with respect to the work piece

and the applied magnetic field could be

studied. And perhaps, arrays of coils or arrays of other types of detectors may be

useful for rapid scanning. Several people

have pointed to the need for that. You could also have other detectors, not just

the little coils that we normally use. Some other detectors may lead more natu- rally into an array configuration than the

coils. I know that a lot of work is going

on in arrays for ultrasonic testing, and I do not see why we would be prohibited from

doing this in eddy current work.

We have heard mentioned the use of

computers for signal processing. There is

the possibility of using computers to not only analyze the data, but also to control

what kinds of data we measure. And that,

again, would tie in with the array; they

could be- used with fast scan systems.

Computer-controlled ultrasonics systems

are already making great progress in this

regard.

The display could range from a red

and green light for some types of

operators, to a very complicated type of

display if it was for a scientist in a research lab. I would hope it would not

bother him too much how complicated the

pattern looked.

I can see some interesting work being

done on developing imaging type displays

to go along with the arrays, or with the

rapid scanning, where you could display

pictures of defects. I would think

whether we like it or not, most people,

non-specialists especially, really like to

see pictures of a defect or flaw and that

is the thing that is coming in this field

as well as other fields.

Multi-frequency techniques and the

pulse techniques are both options that

people are developing and using, and it

appears that more work can be done there.

I was especially interested in Professor

Waidelich's little equation. When you

get a good equation from experimenting,

one which theoreticians have not derived,

it stimulates theoretical work. From the new theoretical work, in turn, you can expand the experimental investigations.

So, this is a nice area to look into

almost immediately.

Eddy current testing in hostile environments presents special difficulties.

Several people brought up the problem of

testing hot items. This also could apply

to cold items, if you happen to have some work in Alaska. Either extreme of temperature will affect test results. With

respect to hostile radiation type environments, there appears to be considerable

progress from the talk given yesterday,

especially the device that Clyde Denton

talked to us about.

Other methods are very interesting to

me, such as the microwave technique and

the phenomenon that Professor Mc Master

pointed out, triboelectricity, materials

emitting electromagnetic waves spontaneously, similar to acoustic emission.

I would like to have some discussion

now, and in the discussion, consider your

144
own practical needs. Where could some scientific work be done that would be

useful to you?

Mr. Brown : At the risk of sounding

too simple, I would suggest that a significant contribution could be made if you

figured out how to make a reproducible

hole in the side of a tube. There are many types of standards, paper standards

that call for a hole of one form or another, 20 percent or 50 percent through

the wall or all the way through. But I am not convinced, in fact, I am very insecure

about the reproducibility of some of these

holes from one piece to another. The

boiler code calls for a flat bottom hole

to go part-way through, and some of the

other standards call for holes all the way

through. But many people use holes for

various purposes, and I do not think those

holes are reproducible enough. I suspect

that NBS could do something that would

help people manufacture reproducible holes

so the data was more comparable back and

forth across the country.

Dr. Green : I assume you mean by a hole, not only the hole itself, but what

would happen to the material surrounding

the hole when the hole is made?

Mr. Brown : Right. This would include deburrings on the inside of a tube,

which could be a real problem.

Dr. Green : Perhaps even the residual

stress?

Mr. Weismante l: Wouldn't you want to start by putting holes in flat plates

first? Eventually, you have to get to a radius effect, but you have to start with

something basic and then build up the

mechanism to get into the more involved

three-dimensional structures at different

radi i

.

Mr. Wehrmeister : I was just going to suggest SDN-243 which is for copper inspection, primarily copper tubing inspections.

It offers some guidance to drilling holes

and filing notches in material, and also

some tolerances which people in that

particular industry have been living with

for a good number of years.

Mr. Berger : I do not want to turn

off this discussion, but I would like to ask a question. Is there a need for

greater theoretical work to go along with

putting holes into flat plates or tubes or

whatever to better understand just what is happening as far as the electromagnetic

fields are concerned?

Dr. Mc Master : Could I respond with

a suggestion for your organization? I feel one of the problems is that if you

take all of these problems in one "mishmash," you are going to have to have

a Latin squares statistical experiment to figure out what did what. If we divide it

in sequence in terms of fundamental phe- nomena, we can have many different groups

working on various aspects. So, let me suggest this division of the problem, and

it is only a suggestion.

The first area of study would be the

magnetizing coils and their fields and

anything that has to do with magnetizing

coils and their fields, including how

these fields are modified when they are in

the presence of a test object. This is a field which could be handled by a group of

people. It involves a portion of these

problems, and involves fundamental theory

and related analyses.

A second field of study would be the

eddy currents. It is perfectly possible

in an eddy current instrument to wipe out

the entire coil signal with electronics,

and then measure nothing but eddy currents. It seems to me, we should study

the eddy currents. It is easy to do in

the constant current .instruments, if you

abolish the coil field and forget it from now on and go to the eddy currents and

their distribution in the metal, for all

the different shapes and/or frequencies

that affect distribution and/or defects,

etc. That could be done possibly by

techniques like we saw demonstrated

earl ier.

A third level is the magnetic field

created by the eddy currents. This is a different problem from the eddy current

distribution. The eddy current magnetic

field is superimposed in space and with

nonferromagnetic test objects we have seen that superposition applies. Thus, we can analyze it independently of the magneti zi ng coi 1 f iel d.

And, finally, the detector response

to the induced field. Since the detector

responds in a uniform way to its magnetizing coil field in the absence of a test object, the induced field response can be

considered independently. Obviously signal analysis and interpretation would be

another area of endeavor.

145
Now, the only reason I suggest this

classification is the obvious fact that

you can divide the job into four parts or five and each could be thoroughly explored

independently of the others. And maybe

when you put the pieces together you have

a coherent program.

Mr. Libby : One comment. I would

join the first two areas since current

distributions are very definitely a function of the coil geometry.

Dr. Mc Master : Yes, there is no question about that. But if you subdivide

and conquer the pieces, you may be able to analyze the relationship between these

pieces. If you put them all together, the

problem gets too complex, at least for my

mind.

Mr. Weismantel : I think the way Dr. Mc Master has organized the problem is a very logical way to go. But, I would like

to see coming out of this whole thing some computer modeling which would give us a much more valuable tool, more powerful

tool for the application of the process.

The problem I face is characteristic of

many people. A lot of applications of

eddy currents that are successfully used

are volume testing of parts with a similar

geometry.

For example, in a turbine engine you

might have 30 different shapes that are unique to whatever the component is that

has to be inspected. These vary in alloy,

and they vary in electrical and thermal

conductivity characteristics. We must

have a way of building up the theory so that we do not have to do everything by

empirical means. We have to know how to handle a radius, and determine what the

sensitivity is for finding a flaw in that

radius relative to the sensitivity on a flat plate. This requires a further

extension of Maxwell's equation and a development of a scientific approach to the subject.

Mr. Titland : In the area of displays, you mentioned two things, either a red and green light, or a picture. But I think there is a third item that should be

considered in displays, that is the standard with holes and defects which checks

the sensitivity of the instrument. For

example, the inspector who comes along

when you are testing pressure vessels

likes to see that the standard will truly

check the functioning of the instrument.

The reason I mention this is there has

been some talk about electrically calibrating detectors. I think that is okay

and should be done, but let's not forget

about the actual standard.

Dr. Green : Any other suggestions?

Mr. Brown : This is a negative

suggestion. I agree with all these lofty

ideals and have for years, like everybody

else. But I caution NBS not to take on the world. Do not try to do it all. We

have all found that it is not practical.

Do a good job in the standards field

first; do not try to expand into

everything.

Mr. Mester : In response to some of

Bob Mc Master's suggestions, the idea

occurred to me, and other people have

mentioned it in some of the previous

discussions, of using taped data. In my

company, we are working to correlate

responses from our equipment to information gathered about actual defects

in the material which are determine by

other methods.

What we do is set our instrument

condition to take the run data and running

the instrument at a particular setting,

for instance. This is a one-shot test

since the material is hot, you never get a chance to run it again. It can never be run through under the same conditions, but

the data is now on tape. You go back and- actually put the tape data back in again,

changing time constants and levels, reject

levels, to force the conditions to fit

what the correlations should be.

What I think may be appropriate when

considering how my equipment compares with

the other fellow's equipment, is producing

on magnetic tape, for instance, a signal

which is indicative of, or representative

of, some standard defect. Of course, the

information on the tape is determined by

environmental conditions, the size of the

coil, the field, and many other things.

But I am saying you have established

at least some reference. This tape can be

given to people who would feed it in at some point in their equipment for testing.

Maybe their signal has a lot of noise and

they are trying to determine just how

capable the equipment is of handling that

background noise. This may be helpful

when everybody is working with the same type of signal. They have the signal

input. What their output is, is determined by a black box. I do not think

146
you can regulate and say everybody should

have the same black box. But if we started with the same hole and the same applied field and we have the same eddy

current pattern, we are looking at the same output. The rest of your system I think you have to consider separately,

and this may be one way of doing it.

National Bureau of Standards Special Publication 589. Proceedings of the Workshop on Eddy

Current Nondestructive Testing held at NBS, Gai thersburg , MD, November 3-4, 1977. Issued

January 1981

.

NEW DIRECTIONS FOR STANDARDS CONCLUDED

FROM WORKSHOP SESSIONS

Norman B. Belecki

Electrical Measurements and Standards Division

National Bureau of Standards

Washington, DC 20234

In any industry, the standards that

are used for quality control and for other

purposes may have many forms. If the

standards that are used do not attack the

primary measurement problems of a specific

industry, the result is wasted time, money,

and unresolved measurement problems. If

the standards that are used do not directly relate to the measurement problems that are faced, there is no reason

for their existence. From the preceding

talks and discussions of the workshop, I have made a list (fig. 1) of areas in

which standards or calibration services

might have a direct impact on industry.

(1) Conductivity Standards

Calibration Service

Standard Reference Materials

(2) Terminology

(3) Performance Standards

e.g., Probe Characteristics

(4) Reference Data

(a) Metals

(b) Comparison of Methods

(c) Geometrical Effects

(5) Tutorial Material, to bridge gap

between theory and practice.

(6) Measurement Methods,

(7) Standards and Recommended Practice

for Cladding Measurement.

(8) Ferromagnetic Standards

(9) Sensitivity Tests for Equipment

(10) Round Robin Tests with Standard

Defects.

Figure 1. Area for standards development.

Before starting the discussion on standards for eddy current testing, I would

like to raise several questions about our

own present program, i.e., conductivity

standards. Is an electrical conductivity

service, either a calibration of customers'

conductivity samples or the issuance of

electrical conductivity SRM's important to the eddy current work now being done in

industry? If this type of standard and

calibration service is important, what are

the limits, in percent IACS, that should

be offered? At present, we are thinking

of working in the area of 3 percent to 100

percent IACS. But should we go lower than

3 percent and if so, how low should we go?

A final question related to the conductivity standards is the range of frequencies that are important in the testing of

samples. Much of the commercial eddy cur- rent equipment functions at 60 kHz. Should we limit ourselves to testing at this frequency? In the testing of other

electrical parameters, I have found that

when one of the parameters is limited in

range, there is a tendency to restrict

advances which could be made in areas that

are peripheral to this parameter.

These are a few of the questions re- lated to eddy current standards that I think need answers. Now, we would like to open the floor to discussion and questions

you might have about eddy current standards, what standards are needed now, and

what standards will be needed in the

future.

Di scussion

Mr. Weistmantel : Why limit the con- ductivity standards to 3 percent IACS?

Why not go lower?

Mr. Belecki : That is what I am trying to get at; how low should we go?

That is the question I am raising.

Mr. Weismantel : Go down to as low a conductivity as we have commercially

available materials. I do not know how

low that would be.
Mr. Free : I had a conversation with

someone from the steel industry several

weeks ago, and I do not know whether this

information is accurate or not. He stated

that for many types of your lower con- ductivity materials, one percent or below,

eddy currents really do not give you much

useful information. You do not search

with them; you cannot tell the difference

between heat treats. It does not give you

the needed information.

Dr. McMaster : I do not agree. I would hate to take a piece of steel and

try to analyze its contents, including

residual things like gases with eddy currents. But, when you get down to titanium

or something like that, you have a chance.

I would suggest we have missed a very

valuable area down there, far more than we

know. Although these are exotic, costly

five dollars-a-pound materials used in critical applications, where do you have a greater need? I would encourage going all the way. I would like to see you go down

to 100th of 1 percent with conductivity

standards.

Mr. Brown : You can sort graphite

that way and it is 1000 micro ohm

centimeters.

Dr. McMaster : And finally, when you

come to graphite fibers in a matrix, you

are going to establish orientation, packing density, etc. , by eddy currents. By

taking directional magnetic fields, you

can get a pretty good idea of the angle of

orientation.

I think there will be a great deal of

engineering moving out of straight metals

into composites and refractory materials,

which by the way become conductive when

they get hot enough. Glass is a beautiful

conductor at higher temperatures.

Mr. Belecki : I was wondering if

there were any other geometries that would

be desirable in conductivity standards.

If so, what would they be?

Mr. Wehrmeister : I am just guessing,

but I would think foil, a very thin material, would be of interest as a conductivity standard. And that, of course, is

going to have a very large effect on the

range of frequencies that you are talking

about.

Dr. McMaster : On the other hand, the

upper range of frequencies should be in

the microwave range and beyond. In the

future, we will be interrogating refractory

coatings on metals by impedance mismatch

reflection signals and their phase or am- plitude. Where it is possible to go up through that range of frequencies it would

be very helpful

.

Mr. Blew : What kind of correlation

would your samples have with what NBS is doing with wafer resistivity samples?

Mr. Belecki : I think we would have

to run experiments to be sure that our measurements are consistent throughout

thei r range.

Mr. Free : This brings up a point. I do not know whether it should be discussed

here or not. I wanted to raise it this

morning, but we did not have time. The

idea of the percent IACS scale for con- ductivity-is it meaningful when the only

time we use it is when we do a cal ibration?

Anytime we are doing a real honestto-goodness measurement, we are talking

about ohms, units of length, etc; we are never talking about percent IACS except as

a mythological beast.

Dr. McMaster : It is used in the

copper industry for electrical copper

only.

Mr. Jones : The aircraft industry

uses it for sorting incoming material.

Dr. McMaster : Only because the

meters have an IACS scale.

Mr. Jones : It is a workable system. We did not like it either when we started

making the standards, but it does work.

Mr. Endler : There are many com- mercial and military specifications about

IACS.

Mr. Belecki : I think that any change

of units would probably have to be done

gradually as these specifications came up for review, to use both, and then even- tually remove the IACS. That is the

normal way of doing things.

Mr. Blew : On the sample geometry

are you going to define criteria there, or

is it going to be up to this group to set

the standard geometry, thickness, and so on?

Mr. Belecki : One of the things that

we hope to do when we get around to con150
sidering the SRM's and what form they

should have, would be to get together with

all the instrument manufacturers and dis- cuss the standardization possibilities in

that area. I think it is almost crucial

in order to get the optimum performance

out of these standards, and I think that

the spill -out of that kind of a meeting

would have an effect, too, on what we would say for criteria on samples submitted to NBS for calibration.

There is an analogy to our regular

calibration program for standards. It is

clearly written in the SP-250, which is

the publication that describes our ser- vices, that we will not accept instruments

for calibration or standards for calibration if they are not serviceable. The

same kind of general statement can serve

for conductivity samples.

I think that as time goes by all that

kind of thing will be ironed out as well.

This problem is analogous to that of

standard resistors. Standard resistors

generally, especially those designed for

oil immersion, have mercury contacts and

certain dimensional characteristics and

our equipment is designed to accommodate

all of those kinds of variables. I assume

that that kind of accommodation will be

reached.

Mr. Blew: Do you have an idea of the

fee, as compared to your standard

resistors?

Mr. Belecki : No. The only thing I would say is that there is a good likelihood that all the measurement equipment

is going to be automated.

Congress requires us to recover the

cost of carrying out the operation; not necessarily of establishing the capability, but carrying out the operation.

Mr. Blew: What would be an ap- proximate price? What is a standard

resistor calibration costing now?

Mr. Belecki : It could be anything

from $100 to $200, someplace in that

range, depending on the work involved.

But I do not think this would cost anything like that.

Dr. McMaster : I get a vague impression that you are expecting to use as standards commercially avaliable metals

and alloys, materials that suit your

needs. At first glance, that seems like

the hard way to go about it. Wouldn't it

be far simpler to start with a relatively

pure material like aluminum, and progressively alloy in copper to cover that range

from 100 percent down to 60 percent down

to 38 percent or so, and do this in a con- trolled experiment so you can build them

to demand? And in fact, if you had a method of hot measurement of the conductivity, you might even tell when you are there, very much like you do in steel.

Mr. Belecki : Yes, we have talked

about a number of such things.

Dr. McMaster : You could add phosphorous to copper to get the rest of the

scale. And then finally, a third suggestion would be for those things that are somewhat different conductivity per unit

square imitating thin foils. The tin oxide

coating, a nonelectrostatic coating, they

are Nisa coatings made by Pittsburgh Plate

Glass, can be laid down on glass. You can put it on glass, quartz, or ceramic so you

can have a stable base, you spray on tin

fluorite, methanol and so on solutions at maybe 1100°F, 1200°F or higher temperatures, and it comes out with a glossy,

chemically resistanct, extremely adherent,

and, as far as I know, longtime stable

coating. This is what is used on the windshields of jet aircraft.

Mr. Belecki : You spoke of these

things Wednesday night.

Dr. McMaster : Yes. I would like to suggest that artificial standards are far more easily prepared than finding a lot of

aluminum that is consistent in its con- ductivity, because they do not make aluminum specifically for its electrical con- ductivity except for certain conductors.

If you make your own you could adjust it

and possibly have a feedback control

device so that when you approach the

desired conductivity, you slow it down or

stop. You could make many of them more precisely, and you could cover everything

in uniform steps instead of having these

funny steps that we have now.

Mr. Free : This was another area I wanted to bring up this morning and did

not. Is it better to make ideal conductivity standards, more of what you are talking about or, is it better to come out

with standards which are really the most common al loys?

Dr. McMaster : The point is these are used to calibrate instruments. If you are

151
calibrating an instrument, you would like

to know in what range or percentage of

full scale, it is accurate. The standards

are never actually used in a comparison

coil to detect a matching alloy. It is an instrument calibration standard.

Mr. Free : Where I would see the handiness of one of the aluminum alloys, is

that it can be carried right to the industrial setup, and you do not have to worry

about corrections for temperature. You

are zeroing in on that alloy, so to speak,

and you are comparing it with a lot of the same alloy you know is in your warehouse.

Dr. McMaster : Those are secondary

standards.

Mr. Belecki : The other advantage in

the alloys is if a person has a lot of

measurements on exactly the same metal

sample, that will allow him to apply differential measurement techniques, rather

than trying to make an absolute conductivity measurement, and raise the sensitivity several orders of magnitude because

of that.

Mr. Richardson : Are you going to

tell us how to make the measurements on a conductivity standard?

Mr. Belecki : I think that would

probably be useful. The area of reference

data is another area that we do not have

any immediate plans in, but it is some- thing that might be useful for various

people.

Mr. Brown : Those of us who have

looked in 10 to 100 books for the values

of resistivity, would like to have one list with all the materials on that list.

Mr. Schwarz : How about some reference data on permeabilities of various steels?

Mr. Weismantel : Reference data does

not have to be limited to eddy currents

you know. Velocity of sound is an important area.

Mr. Wehrmeister : It was mentioned

yesterday that vendors have to run ROI's,

returns on investments, to see if it is

feasible to do something for general

industry. I do not, somehow, suspect that

the Bureau runs return-on-investment

studies and it might be possible to supply

things to industry that could not be

normally supplied by an industrial vendor.

Mr. Belecki : I think that is true.

Mr. Wehrmeister : Especially in cus- tom-made standards, for example.

Mr. Belecki : I think that is surely

true. We have done that in a couple of

cases. In my own field, for example, an electronics company was having problems

verifying measurements on the production

line for ladders used in A/D converters,

and we built a special transportable

standard so that we could directly test

that process with them.

That brings up another thing that

your talk implied; we have methods of ser- vicing customers in our regular calibration areas via techniques similar to the

round robin method that you talked about.

It might be a very useful approach especially in relation to those areas where

people have a lot of data, but do not know

how to correlate it with data that some- body el se has.

Mr. Reinhardt : There is another role

for NBS. I am always looking for uncon- cerned third parties; a team that might

have some interest, personal or professional, in looking at a test a certain

way.

Mr. Belecki : Considering measurement

methods, one of the things I thought of

that would be useful would be a method of

testing the sensitivity of measuring

equipment. That seems to be somewhat in

question in some of these various areas. The work at Battelle Northwest that was described yesterday by Mr. Davis would be

a possible way to pursue that type of

test.

Mr. Brown : Along the lines of a measurement method, there is a significant

degree of uncertainty in describing the

many variables in an eddy current test; how it is set up, how stable it is, what

the results are and what do they mean

etc.; probabilities and statistics are involved. I recognize there are practical

and statistical methods for handling these

things; but, from the users' standpoint

they usually are too complicated for practical application.

Mr. Belecki : We have the same problems in the standards area.

Mr. Brown : The Bureau of Standards

could make a great contribution by

evolving some simple--let me say that

152
three times—simple, simple, simple--

statistical techniques, for evaluating the

uncertainties involved.

Mr. Belecki : I suspect that there

are large numbers of measurements made on clad surfaces, and I wondered what kind of

standards might be useful in that area?

Mr. Jones : Mil 1537 covers a little

bit of that. I have been rewriting it. We are also putting definitions in, which

comes under terminology. Maybe NBS, would

be interested in seeing what we define as conductivity in primary standards, sec- ondary standards, etc., so we could get

some agreement at the beginning, because

it is still being written.

Mr. Weismantel : Do you really want

to get into the areas you have listed

there? In other words, standards for cladding, standards for defects? You have

tubings, you have other things. It seems

to me that there are so many ungodly com- binations of things, alternatives.

Mr. Belecki : I think that the way I would have to answer that would be to say

that we would get into an area if it were

useful to get into, and were within the

range of our resources to do so. I understand what you are saying. We could not possibly supply people with artifacts or calibration service for standard flaws for

every possible conceivable application.

Mr. Weismantel : A problem that we

have is the fact that there are not any

good standards available that can be used

to measure relative detection efficiency

to determine when there are improvements

or lack of improvements in a process due

to advancements in the process technology.

There is no base to say that one instrument or method is better than another. The

T-crack blocks that I discussed in my talk

is our attempt to do this internally.

They are our own controls to get a measure

of where we are. But it would be extremely

useful if somewhere there was a grand

master series of buried defects. No EDM notches or anything like that, but subsurface flaws, that could be used to gauge

process efficiency. There are techniques

in which you can produce subsurface flaws

of different sizes in character and orientation. And natural-looking flaws, not

EDM notches and things of this sort.

Mr. Belecki : Well, don't you think

there is some possibility of being able to synthesize those kinds of things artificial ly?

Mr. Weismantel : That is what I am saying, you can artificially synthesize

them.

Mr. Ammirato : That depends on what

object you are after. On the one hand,

you have to calibrate the instrument to make sure that is working all right. On

the other hand, you are trying to get some

idea of what the actual flaw size is in

your part. If you have them varied, you

will not know what size they are.

Mr. Weismantel : You can develop a technique where you get a good prediction

of what the size is. We have gone through

it, but not being farsighted enough, we cut these things up, something which you

would like to avoid so that the test

pieces are preserved. Then you can always

go back and measure relative detection

efficiency. The defects have to be in

large enough quantity to give you some statistical appraisal of what you have

got.

Mr. Titland : Two comments on this:

First, I think the Welding Institute in

England has developed a technique where a flaw is placed inside the material by

machining in pieces, and then putting the

pieces together in a vacuum. I think this

technique may be available. Secondly, when

we make a standard it could be used not only for eddy current, but also for

radiographic and ultrasonics.

Mr. Belecki : That is right.

Mr. Weismantel : So that you could

compare the efficiency of the different

processes.

Mr. Reinhardt : This, to me, is a very critical area, this simulation of

real defects and keeping them in an ar- chival place. It opens up another area which is a computer learning method. If you have them in some central place where

they are never taken apart, this allows

people with the difficulty to go there.

We have a deficiency of seeing real or simulated flaws in our industry, because

the metallurgists get there first and take

them apart.

Mr. Weismantel : That is the only way

they know what size they are.

Mr. Belecki : We only have a couple

of more minutes. Synthetic standards like

those Tom Davis talked about yesterday

could be used in a large variety of ways,
both as ways of testing instrumentation

and ways of augmenting some of the theoretical things that we talked about

earl ier.

One of the things that we were talking about at lunch time, which we thought

might be useful, is the characterization

of a measurement system component. Perhaps

it might be worthwhile to have some sort

of performance specifications or something

of that order for probes and pickups.

Mr. Weismantel : The ferrites that go

into them, start with them because they

have a large variability.

Dr. McMaster : And the ceramic bases

for the coil forms tend to distort. There

should be a ceramic with zero temperature

coefficient of expansion, wound with Invar

or something, which has a small temperature coefficient.

coming down the power line on your mea- suring apparatus. Do you have a lot of

problems in that regard? We have had a fair amount of input from companies,

especially those making medical instruments, that want some kind of standardization in that area for liability purposes. Right now if they make an instrument that is totally free from that kind

of effect, they cannot be competitive.

Mr. Mester : I was just going to say

that that is a very big problem. When you

buy equipment in the lab it works fine.

You put it in the field and you have problems; transducers probably aren't chilled;

ground loops exist, you have problems with

noise coming in on the line, even if you

have taken pains to separate your lines.

It is a very big problem, and I do not want to go into it a lot. But it is an area which has been neglected.

Mr. Belecki : Any other comments?

Mr. Blew : Not on methods, but on the

instrumentation itself as far as the performance is concerned. It seems from what

I have gotten, that it is varied as far as the sensitivity and the noise characteristics are concerned. You have been more

or less talking about electronics in a black box. I looked at NBS Technical Note

865 from the meeting in 1974 and saw sources and standards, and also on the

devices such as A/D D/A, and I imagine

stability of analog dividers and so on. I was wondering, has any of this been re- solved since the 1974 meeting?

Mr. Belecki : Many of the things that

are mentioned in there are being worked on

in the program of the Electrical Instruments Section that Barry Taylor discussed

yesterday. They have done a fair amount

in the way of terminology and data con- version work or at least they are right in

the middle of doing that. There are, I think, three committees, IEEE committees

and ANSI committees, that members of the

section are working on. And I think that

especially in the data conversion area, and in the area of microprocessor and

instrumentation interfaces, there is work

being done.

The noise problem has not been addressed yet, and that brings up another

problem that we have heard about from the

general instrumentation and test equipment

industry. I wonder what comments I might

get from you about the effects of noise
ACKNOWLEDGMENTS

NBS would like to give special thanks

to Mr. J. R. Barton of Southwest Research

who presented a paper on eddy current

testing in the transportation industry.

This paper was not available at the time

of publication. Also appreciated were presentations by Mr. Rudy G. Hentschel

,

Hentschel Engineering, entitled "Lets Not

Forget Standards for Ferrous Metals," and

a review of the work done at Battel le

Northwest on Simulating Tubing Flaws by

Mr. Tom Davis of Battel le. The former was

not available at the time of publication

and a complete discussion of the latter

topic can be found in Materials Evaluation, "Calibration of Eddy Current Systems with Simulated Signals," by J. C. Crowe, Sept. 1977, Vol. 35, Number 9, p. 59.

We would like to thank ASTM for permission to print the tables which appear

at the end of Dr. McMaster's paper. These

tables are from ASTM Special Technical

Publication No. 112.

Our sincere thanks to all who participated in the workshop.

155

LIST OF PARTICIPANTS

Frank Ammirato

General Electric Co. M&T Lab, Bldg. 28, Rm. 450

1 River Road

Schenectady, NY 12345

J. R. Barton

Southwest Research Institute

P. 0. Drawer 28510

San Antonio, TX 78284

Winthrop A. Baylies

ADE Corporation

127 Coolidge Hill Road

Watertown, MA 02172

Robert Betz

Pratt & Whitney Aircraft

Department 983

K Building

East Hartford, CT 06108

Austin R. Blew

Lehighton Electronics, Inc.

R. D. 1, Box 374

Lehighton, PA 18235

Creighton Booth

Bethlehem Steel Corp.

Bethlehem, PA 18016

Russell L. Brown

Westinghouse Hanford Co.

P. 0. Box 1970

Mail Stop W-A132

3707 C Building

Richland, WA 99352

Don Bugden

Magnetic Analysis Corp.

535 S. Fourth Avenue

Mount Vernon, NY 10550

Carl E. Burley

Reynolds Metals Company

4th & Canal Streets

Richmond, VA 23261

Caius V. Dodd

Oak Ridge National Laboratory

Metals & Ceramics Div.

P. 0. Box X Oak Ridge, TN 37830

Tom Davis

Battel le Northwest

PS Bldg. 3000 Area

Richland, WA 99352

Clyde Denton

ZETEC, Inc.

P. 0. Box 140

Issaquah, WA 98027

Thomas F. Drumwright

Alcoa Technical Center

Alcoa Center, PA 15069

John F. Endler

Qual ity Assurance

The Anaconda Company, Brass Di

P. 0. Box 830

Waterbury, CT 06720

Robert H. Foxall

Wean United, Inc.

P. 0. Box 751 1985 N. River Road

Warren, OH 44482

John Gleim

Naval Ship Engineering Center

Code 610D

Hyattsville, MD 20782

Robert Green

Johns Hopkins University

Mech. & Matls. Science Dept.

Baltimore, MD 21218

Rudy G. Hentschel

Hentschel Instruments, Inc. 2505 South Industrial Highway

Ann Arbor, MI 48104

Howard Houserman

Westinghouse NDS

RSD Center Bldg. 701

P. 0. Box 2728

Pittsburgh, PA 15230

Art Jones

Boeing Aerospace Company

Org. 2-4807, MS 2P-10

P. 0. Box 3999

Seattle, WA 98124

Tyler W. Judd

Republic Steel Corporation

Research Center

6801 Breckville Road

P. 0. Box 7806

Independence, OH 44131

John Kol lar

Balteau Electric Corp.

63 Jefferson Street

Stamford, CT 06902

157
Lawrence Lagin

Grumman Aerospace Corp.

Quality Control Laboratory

Plant 10 Bethpage, NY 11714

A. L. Lavery

DOT Transportation Systems Center

Kendall Square

Cambridge, MA 02142

Hugo Libby

2302 Frankfort Street

Richland, WA 99352

Tracy McFarlan

Magnaflux Corp.

7300 West Lawrence Avenue

Chicago, IL 60656

Patrick C. McEleney

U. S. Army Materials & Mechanics

Research Center

Arsenal Street

Watertown, MA 02172

Robert C. McMaster

6453 Dublin Road

Delaware, OH 43015

Michael Mester

U. S. Steel Corp.

Research Laboratories

125 Jamison Lane Monroeville, PA 15149

Richard B. Moyer

AISI Committee on Nondestructive Testing

and Inspection Materials

Carpenter Technology Corp.

Research and Development Center

Reading, PA 19603

Noel Poduje

ADE Corporation

127 Coolidge Hill Road

Watertown, MA 02172

Eugene R. Reinhardt

Electric Power Research Institute

3412 Hi 11 view Avenue

P. 0. Box 10412

Palo Alto, CA 94303

Guy D. Richardson

Detek, Inc. 6807 Cool ridge Drive

Camp Springs, MD 20031

Walter Rolfe

Magnaflux Corp.

7300 West Lawrence Avenue

Chicago, IL 60656

Allen Sather

Materials Science Division

Argonne National Laboratory

Argonne, IL 60439

Ronald Schwarz

Manufacturing Development Bldg. B GMTC

Warren, MI 48090

L. Erik Titland

Baltimore Gas & Electric Co. Westport Metallurgical Laboratory

P. 0. Box 1475

Baltimore, MD 21203

Donald Waidel ich

University of Missouri

Dept. of Electrical Engineering

Columbia, M0 65201

A. E. Wehrmeister

Babcock & Wi Icox Co. Lynchburg Research Center

P. 0. Box 1260

Lynchburg, VA 24505

E. E. Weismantel

Quality Measurements

Mail Drop E45

General Electric Co. Aircraft Engine Group

Cincinnati, OH 45215

National Bureau of Standards Attendees

Dr. L. H. Bennett

Mr. H. Berger

Dr. G. Birnbaum

Mr. N. Belecki

Mr. G. Free

Dr. A. Kahn

158
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U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS SP 589

2. Performing Organ. Report No. 3. Publication Date January 1981

4. TITLE AND SUBTITLE

Proceedings of the Workshop on Eddy Current Nondestructive Testing held

November 3-4, 1977 at the National Bureau of Standards, Gaithersburg, MD.

5. AUTHOR(S)

George M. Free, Editor

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

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WASHINGTON, D.C. 20234

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Same as No. 6

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JJ Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT ( A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The proceedings of the Eddy Current Nondestructive Testing Workshop held at NBS

in November, 1977 contains papers related to all areas of eddy current testing. A

historical overview of the discipline from its inception until the present is given.

Other papers discuss the use of eddy current testing in the primary metals industry

(both ferrous and nonferrous metals), the use of eddy currents for the sorting of

metals and for defect detection, the state-of-the-art in eddy current instrumentation,

and the use of signal processing in the analysis of eddy current signals. The development and use of eddy current standards is discussed as well as several of the newer

areas of eddy current development, i.e., mul ti frequency and pulsed eddy current

techniques. .

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

Conductivity; defect detection; eddy current test; multifrequency; nondestructive

[ testing,

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